BUG: fixed race condition setting m_ANTSAssociate

Found with thread sanitizer (TSan) running the `itkANTSNeighborhoodCorrelationImageToImageRegistrationTest` unit test.

There was already a `m_ANTSAssociateOnceFlag` flag for use with `std::call_once`, so I just used that in this place too (it's already used elsewhere).

Partial backtrace from TSan:

```
WARNING: ThreadSanitizer: data race (pid=79176)
  Write of size 8 at 0x000108f02240 by thread T2:
    #0 void itk::ANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>, itk::ANTSNeighborhoodCorrelationImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreadedExecution_impl<itk::ThreadedIndexedContainerPartitioner>(itk::IdentityHelper<itk::ThreadedIndexedContainerPartitioner>, itk::Index<2u> const&, unsigned int) itkANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader.hxx:118 (ITKMetricsv4TestDriver:arm64+0x1004bfe50)
    #1 itk::ANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>, itk::ANTSNeighborhoodCorrelationImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreadedExecution(itk::Index<2u> const&, unsigned int) itkANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader.h:235 (ITKMetricsv4TestDriver:arm64+0x1004bf9c0)
    #2 itk::DomainThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreaderCallback(void*) itkDomainThreader.hxx:123 (ITKMetricsv4TestDriver:arm64+0x100259b6c)
    #3 std::__1::future<std::__1::invoke_result<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>::type> itk::ThreadPool::AddWork<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&)::'lambda'()::operator()() const itkThreadPool.h:92 (ITKMetricsv4TestDriver:arm64+0x1007d3228)
    #4 decltype(std::declval<void* (*&)(void*)>()(std::declval<itk::PoolMultiThreader::ThreadPoolInfoStruct*>())) std::__1::__invoke[abi:v160006]<std::__1::future<std::__1::invoke_result<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>::type> itk::ThreadPool::AddWork<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&)::'lambda'()&>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&) invoke.h:394 (ITKMetricsv4TestDriver:arm64+0x1007d31a4)

  Previous write of size 8 at 0x000108f02240 by thread T14:
    #0 void itk::ANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>, itk::ANTSNeighborhoodCorrelationImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreadedExecution_impl<itk::ThreadedIndexedContainerPartitioner>(itk::IdentityHelper<itk::ThreadedIndexedContainerPartitioner>, itk::Index<2u> const&, unsigned int) itkANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader.hxx:118 (ITKMetricsv4TestDriver:arm64+0x1004bfe50)
    #1 itk::ANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>, itk::ANTSNeighborhoodCorrelationImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreadedExecution(itk::Index<2u> const&, unsigned int) itkANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader.h:235 (ITKMetricsv4TestDriver:arm64+0x1004bf9c0)
    #2 itk::DomainThreader<itk::ThreadedIndexedContainerPartitioner, itk::ImageToImageMetricv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double, itk::DefaultImageToImageMetricTraitsv4<itk::Image<double, 2u>, itk::Image<double, 2u>, itk::Image<double, 2u>, double>>>::ThreaderCallback(void*) itkDomainThreader.hxx:123 (ITKMetricsv4TestDriver:arm64+0x100259b6c)
    #3 std::__1::future<std::__1::invoke_result<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>::type> itk::ThreadPool::AddWork<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&)::'lambda'()::operator()() const itkThreadPool.h:92 (ITKMetricsv4TestDriver:arm64+0x1007d3228)
    #4 decltype(std::declval<void* (*&)(void*)>()(std::declval<itk::PoolMultiThreader::ThreadPoolInfoStruct*>())) std::__1::__invoke[abi:v160006]<std::__1::future<std::__1::invoke_result<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>::type> itk::ThreadPool::AddWork<void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&)::'lambda'()&>(void* (*&)(void*), itk::PoolMultiThreader::ThreadPoolInfoStruct*&&) invoke.h:394 (ITKMetricsv4TestDriver:arm64+0x1007d31a4)
```

Modules/Registration/Metricsv4/include/itkANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader.hxx

@@ -113,14 +113,16 @@ ANTSNeighborhoodCorrelationImageToImageMetricv4GetValueAndDerivativeThreader<
                                                           const DomainType & domain,
                                                           const ThreadIdType threadId)
 {
-  /* call base method */
-  /* Store the casted pointer to avoid dynamic casting in tight loops. */
-  this->m_ANTSAssociate = dynamic_cast<TNeighborhoodCorrelationMetric *>(this->m_Associate);
-  if (this->m_ANTSAssociate == nullptr)
-  {
-    itkExceptionMacro("Dynamic casting of associate pointer failed.");
-  }
+  std::call_once(this->m_ANTSAssociateOnceFlag, [this]() {
+    /* Store the casted pointer to avoid dynamic casting in tight loops. */
+    this->m_ANTSAssociate = dynamic_cast<TNeighborhoodCorrelationMetric *>(this->m_Associate);
+    if (this->m_ANTSAssociate == nullptr)
+    {
+      itkExceptionMacro("Dynamic casting of associate pointer failed.");
+    }
+  });
 
+  /* call base method */
   Superclass::ThreadedExecution(domain, threadId);
 }