Add spin_multiplicity as a known quantity

[JonathanSchmidt1]

Adds Spin/Charge to KNOWN_INPUTS (ase integration in another PR)
and a respective test/docs.
_check_outputs also seems to be used for checking inputs with check consistency so I asked claude to add respective checks ...
Not sure if we want separate docs for inputs but i imagine also with support for FlashMD and momenta inputs it might be nice to separate it.

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?

[Luthaf]

Not sure if we want separate docs for inputs but i imagine also with support for FlashMD and momenta inputs it might be nice to separate it.

There is no distinction between stuff that can be used an inputs and stuff that can be produced as outputs, and everything lives in output/<name>.rst for historical reason, and might move with the next breaking change.

[JonathanSchmidt1]

Not sure if we want separate docs for inputs but i imagine also with support for FlashMD and momenta inputs it might be nice to separate it.

There is no distinction between stuff that can be used an inputs and stuff that can be produced as outputs, and everything lives in output/<name>.rst for historical reason, and might move with the next breaking change.

Thank you for the review. Than I will put everything back into outputs.

[JonathanSchmidt1]

@Luthaf I removed the charge part and switched to spin-multiplicity as requested. Please let me know if it looks good now.