fix shape equality check in solver

The proper way to compare shapes of `evaluable.Array` instances is using
`evaluable.equalshape`. This patch replaces two `shape1 == shape2` checks with
`evaluable.equalshape(shape1, shape2)` in the `solver` module.

nutils/solver.py

@@ -614,7 +614,7 @@ def __init__(self, target, residual:integraltuple, inertia:optionalintegraltuple
     if len(residual) != len(inertia):
       raise Exception('length of residual and inertia do no match')
     for inert, res in zip(inertia, residual):
-      if inert and inert.shape != res.shape:
+      if inert and not evaluable.equalshape(inert.shape, res.shape):
         raise ValueError('expected `inertia` with shape {} but got {}'.format(res.shape, inert.shape))
     self.target = target
     self.timesteptarget = '_pseudotime_timestep'
@@ -704,7 +704,7 @@ def __init__(self, target, residual:integraltuple, inertia:optionalintegraltuple
     if len(residual) != len(inertia):
       raise Exception('length of residual and inertia do no match')
     for inert, res in zip(inertia, residual):
-      if inert and inert.shape != res.shape:
+      if inert and not evaluable.equalshape(inert.shape, res.shape):
         raise ValueError('expected `inertia` with shape {} but got {}'.format(res.shape, inert.shape))
     self.target = target
     self.newtonargs = newtonargs