Move from NNPOps to MLIPOps

[mattwthompson]

From OpenMM developers:

NNPOps effectively superseded by MLIPOps

@fjclark might be important for Presto? I'm moving all of this code elsewhere but there isn't a clear timeline for moving it into production

[fjclark]

Thanks for raising this! The main issue for presto was avoiding NNPOps since it wasn't needed for intramolecular fitting and was a massive pain with other MLP deps. So not important for presto but could matter for @JMorado's https://github.com/JMorado/SCALeJ

[mattwthompson]

Input from @lilyminium is that we do need this (having moved the issue to tyff). The authors wrote some docs but not a migration guide, so I will have to look more closely into necessary changes