Pshmem

The `MPIShared` class now accepts pre-existing node and node-rank communicators in the constructor.

When encountering an error, print a message and raise or re-raise an exception. This allows easier viewing of problems that occur in the underlying mpi4py package. It does mean that calling applications must be diligent about handling exceptions on all processes an taking appropriate action to avoid deadlocks at future barriers and collective calls.

Properly free communicators at destruction of `MPIShared` objects. Expand unit tests to include split communicators and `MPI.COMM_SELF`.

Handle the case of slices with None values (e.g. `[:]`).

This PR adds support for using square-bracket slice assignment to the MPIShared class. This syntax is still a collective operation and may be slightly slower than using the `set()` method since it must first do and `Allreduce` to determine which process has a data input which is not None.

This removes the namespace import of the test subpackage, which explicitly imports mpi4py. The pshmem package can now be imported without triggering an mpi4py import. mpi4py will only be used if a communicator is passed which is not None.