Changes needed in support of alchemiscale-fah

[dotsdl]

This PR contains all changes needed for alchemiscale-fah MVP support.

[dotsdl]

@ijpulidos can I get an initial review on this one? I'm not sure we'll need any additional changes in feflow in support of openforcefield/alchemiscale-fah#7, but I'll keep it in [WIP] state until I'm certain of that. A review will still be helpful to ensure there aren't any obvious gaps in the changes here.

[dotsdl]

Actually, I think @ijpulidos is on vacation starting today; can I get a review from you instead @IAlibay?

[dotsdl]

@ijpulidos I think it's safe this is ready for merge! It's unclear to me why the CI isn't passing, however; is this something you have any insights on?

[ijpulidos]

@dotsdl The CI failing is because we have temporarily dropped support for GAFF, but we are still using it in our tests and these are not easy to adapt without some non-trivial amount of work. Thats also why it's working with older python (which install older versions of the dependencies and GAFF that work fine).

The changes in this PR are unrelated to that issue, this should be ready to be merged. LGTM!