Define support policy for OpenMM versions

[ijpulidos]

We probably need to define a policy in terms of what OpenMM versions we support. I am not sure OpenMM follows semantic versioning or any versioning system, but we should probably try to come up with a reasonable policy of what versions we want to support from OpenMM, assuming OpenMM uses MAJOR.MINOR.PATCH convention (which it does so far).

[mikemhenry]

This is a good question, OpenMM doesn't really follow semantic versioning (breaking API changes happen without MAJOR bump). I propose that we support the current release and at a best effort support the n-1 release.

[mainguyenanhvu]

I found that when I use different combination of openmm and openmmtools, it has potential errors like this case of openmm 7.5.1 and openmmtools 0.24.0:

python3.9/site-packages/openmmtools/utils/equilibration.py", line 6, in <module>
    from openmm import unit, OpenMMException
ModuleNotFoundError: No module named 'openmm'

However, when I use openmmtools 0.19.0 with openmm 7.5.1, it works well.

You please show me suitable combinations of these tools, so I can easily work.