Resuming and basic tests with MPI context

[ijpulidos]

While working on #602 it was noted that the issue is only relevant when the simulations are run using 2 or more MPI processes (multiple GPUs). Therefore, we need a way to test similar scenarios in our software, since some of these bugs are MPI-specific.

mpiplus is what we are using for creating and handling MPI contexts, it has a Dummy MPI object here that can be useful for the tests.

[mikemhenry]

This looks useful as well https://github.com/mpi4py/setup-mpi

[jchodera]

If we are already installing mpiplus as part of openmmtools dependencies (which in turn installs mpi4py), we shouldn't need to do anything special to test on MPI other than to run mpirun -np 2 <our normal python CLI here>. The question is how we can have pytest launch MPI tests.

Perhaps the pytest-mpi plugin is needed here? It seems to enable us to mark specific tests to be run inside processes launched by mpirun. See usage here.