import mpi hangs on multi node flux allocations

[mdavis36]

I've seen spheral_ats.py hang when import mpi is called. This happens when building ENABLE_MPI=ON and under a flux allocation with more than 1 node. A workaround using SpheralConfigs to determine the MPI build status of Spheral was impleneted but this should be investigated.

[ldowen]

This issue is resolved by ensuring flux run is used before running ./spheral or ./sphera-ats, even if already in an allocation from flux alloc. Ideally, one should use flux run -N <number of nodes> --exclusive.