forcefields: Standardize MLFF handling and maker naming

[lan496]

Summary

• Introduce _get_standardized_mlff (returns canonicalized MLFF) and hide a private function _get_formatted_ff_name
• Use standardized MLFF in ForceFieldMixin for default calculator kwargs and task docs; remove redundant str(...) casts
• Add mlff property to ForceFieldMixin and PhononMaker
• Update forcefield flow makers to use mlff and remove _get_formatted_ff_name

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[JaGeo]

@lan496 thank you for this first contribution! To me, this looks fine but I will wait for @esoteric-ephemera final approval before merging.

In any case: Could you document the use of the new methods and modified forcefield flows within the documentation or tutorials? That would be highly appreciated.

[lan496]

@JaGeo Thank you for your comment! I've introduced mlff property in the tutorial, fe54e06. I did not modify documentation for force fields because this page does not seem to be a place to describe APIs.

[lan496]

BTW, my motivation of this PR is to enable the use of an externally defined calculator constructors with atomate2.forcefields on top of this change; For example, as follows

   calculator_meta = {
       "@module": "chgnet.model.dynamics",
       "@callable": "CHGNetCalculator",
   }
   maker = PhononMaker.from_force_field_name(
       force_field_name=calculator_meta,
   )

I am implementing this approach in a separate branch lan496@324fe30. I would appreciate it if the maintainers could consider this extension after this PR is merged.

[JaGeo]

@lan496 I will merge this PR as everything is backwards compatible.

I also really like the idea to simplify the usage of external calculators. Thus, I would highly welcome such a PR.

Thank you! Btw, feel free to add your name to the list of contributors in the documentation if you like.

With regard to documentation: we should likely introduce a section on forcefields in the future. I will open a separate issue to not lose track of it.

[esoteric-ephemera]

Thanks @lan496, I will take a look when I'm back in the office from the hoidays. If you want to merge now @JaGeo, please do. We can always put out a release candidate to test changes

[JaGeo]

I merged!

I realized there is a section on forcefields already. @lan496 maybe you can extend this section with the next PR. I guess then usage of such external calculators needs to be documented more anyhow 😃