Update for new OpenMM Namespace

[mikemhenry]

Update to support openmm >= 7.6 namespace, but also support older versions of openmm as well

[mikemhenry]

fixes #1260

[ijpulidos]

Thanks, this looks great!

[mikemhenry]

@jchodera it looks like the CI for Yank has been broken for a very long time, do you know what could be causing these schema errors? It looks like the same errors are showing up in the scheduled CI happening on master
https://github.com/choderalab/yank/actions
So this PR isn't the cause, and it doesn't create any new issues, but if there is an easy fix for the schema errors, it would be nice to get the CI working again!

[jchodera]

@mikemhenry : From memory, I think we've run into cases where updates to cerberus (the tool we use to provide helpful error messages when users specify incorrect YAML schemas) cause things to fail for us. We may just need to pin cerberus for now and sort out the issue later.

Here's an example of a previous instance in which cerberus broke things: #1180

[mikemhenry]

Thanks! It looks like it is pinned to 1.1, but the PR makes it sound like it was updated to 1.2
- cerberus ==1.1 # yank uses buggy cerberus 1.1 behavior as a feature ¯\_(ツ)_/¯
I will play around with that, thanks!

[mikemhenry]

@jchodera you might want to start using this PR to base your yank updates on since it takes care of the namespace changes in openmm