Move YANK's MPI module to Clusterutils

[Lnaden]

The mpi.py module from @andrrizzi in YANK is so amazingly useful, I think we should move it to a more generalized location. After speaking to @andrrizzi, the module could even be reduced to ONLY depending on built-in Python packages and only optionally on mpi4py if present.

I actually think this module is so helpful that it should be made as a possible full standalone and published to the much broader community such as on conda-forge (would need a name change).

As a stopgap, I think moving it here would improve clusterutils as package in of itself, since right now its effectively just a "MPI script builder under certain conditions"

Thoughts?

[Lnaden]

I would add there does not appear to be any such utility on conda-forge right now, searching for terms "mpi" or "cluster"

[andrrizzi]

+1 for clusterutils!

[Lnaden]

The twin issue on YANK is open at choderalab/yank#781

For now the main discussion should happen on the YANK issue since that is where the module currently rests. I'll strike/remove this comment if we decide to migrate it the module here.