[On hold] Force mask

[FNTwin]

Fixes #42

Checklist:

• Was this PR discussed in a issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• Update the API documentation is a new function is added or an existing one is deleted.

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Add the following:

• __force_mask__ as private attribute
• add energy_methods,force_mask , force_methods property methods
• __force_methods__ are now based on energy_methods,force_mask
• Add the correct __force_mask__ to every dataset
• Automatically initialize __force_mask__ to self.__class__.__force_mask__ = [False] * len(self.energy_methods) if empty (aka no forces)

Do not solve or merge as it could lead to breaking change down the line on openMLIP

[prtos]

Should we close this PR Now since it is already included in another one.

[FNTwin]

Present in #45