Enabling compatibility with the latest OpenMM release

[jchodera]

Our recent OpenMM 7.6 release included some namespace changes that look liked they required pinning this repo to OpenMM 7.5.1.

Is it OK with you folks if we propose a pull request that should enable compatibility with OpenMM 7.6 and later (ideally without breaking backwards compatibility as well)?

[gahdritz]

Absolutely!

[gahdritz]

I should add that we inherited this OpenMM version number from the original AlphaFold---it's likely that DeepMind ran into the same issue.