deal with openmm 7.6 package unwrapping

[sroet]

Description

importing OpenMMTools raises warinings about importing openmm from simtk, which make OpenPathSampling's notebook tests fail (unexpected warning in the stderr output)

This updates all imports to the new standard.

I went through grep -r -n "simtk" --include="*.py" -l and updated all references to simtk. For the doc-tests I decided to only include the current advised method of importing openmm. For all real code, I wrapped the new method in a try/except clause that drops back to the old style imports.

Todos

• Implement feature / fix bug
• [NA] Add tests
• [NA] Update documentation as needed
• Update changelogNotable points that this PR has either accomplished or will accomplish.

Status

• Ready to go

[jchodera]

Thanks so much for updating this in a backwards-compatible manner!

@ijpulidos can you review and merge when able?

[sroet]

test failure is identical to master. (and is for an unsupported python, based on NEP 29)

[jchodera]

Thanks! And thanks for pointing out NEP 29---I hadn't seen that before, but we will try to standardize around that!

[ijpulidos]

This looks great, thanks!