Replace HarmonicAngleForce with AngleForce (and likewise for bonds?) in SMIRFF

[davidlmobley]

We need to replace the HarmonicAngleForce SMIRFF tag with AngleForce, which would default to harmonic unless otherwise specified ( to allow the format to support other forms if desired), as per @jchodera discussion with Swope and others at IBM.

@jchodera - presumably we should do the same for bonds as well, to potentially allow support for Morse potentials, etc.?

[jchodera]

Yes, I think we should generalize bonds and angles this way.

We should also generalize NonbondedForce to permit other forms (e.g. Buckingham exp-6, Halgren buffered 14-7, etc.).

[davidlmobley]

We should also generalize NonbondedForce to permit other forms (e.g. Buckingham exp-6, Halgren buffered 14-7, etc.).

Yes. I didn't mention that here, as HarmonicAngleForce and HarmonicBondForce are tags in the format which need changing, whereas the NonbondedForce tag doesn't need changing (I don't think) we just need to change how it's implemented. In other words, changing bonds/angles requires modifying our current code whereas handling other nonbonded forms just requires generalizing/extending. So I figured that was a separate issue.

(I'm working up a PR to the format spec which would address these issues.)

[jchodera]

Could address #42 (versioning) as well!

[davidlmobley]

On versioning - I commented on #42.

[jchodera]

Addressed in openforcefield/openff-toolkit#86