Running unit tests in parallel

I remember we had a discussion about it before but I couldn't find the issue. The problem is when you run unit tests in parallel with mpirun most of them fail with Segmentation violation signal. For PluginPlay and ParallelZone we don't have this problem, so the issue is TW specific.

I encounter this while trying to understand the problem with passing an mpi communicator to madness runtime. I can look into more, but first I wanted to see if this is already discussed somewhere.

Here is an incomplete list of failed tests:

/home/keceli/soft/nwx/TensorWrapper/tests/tensor/buffer/scalar_buffer.cpp:508: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/performance_tests/add.cpp:31: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/ta_helpers/einsum/einsum.cpp:56: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/diis/diis.cpp:48: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/tensor/creation.cpp:315: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/tensor/buffer/detail_/scalar_ta_buffer_pimpl.cpp:32: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/tensor/linear_algebra.cpp:90: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/tensor/linear_algebra.cpp:62: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/diis/diis.cpp:48: FAILED:
/home/keceli/soft/nwx/TensorWrapper/tests/ta_helpers/remove_redundancy.cpp:75: FAILED:

Start by reproducing the failures with mpirun, beginning with the listed tests such as tests/tensor/buffer/scalar_buffer.cpp and tests/tensor/creation.cpp. Compare TensorWrapper's behavior with PluginPlay and ParallelZone, then trace the MPI communicator handoff into the MADNESS runtime. Done means the affected unit tests no longer segfault when run in parallel.