Fix MPMD set bug

[MichaelWolloch]

Summary

Small bugfix for the MPMD vasp input set. The input parameter time_step was added in fabdd27 (it was in the docstring before that), but it was not added as a class attribute, leading to an NotImplementedError raised when trying to convert the object to a dict:

NotImplementedError: Unable to automatically determine as_dict format from class. MSONAble requires all args to be present as either self.argname or self._argname, and kwargs to be present under a self.kwargs variable to automatically determine the dict format. Alternatively, you can implement both as_dict and from_dict.

Major changes:

• feature 1: time_step is now usable and is passed on to POTIM. It defaults to None, in which case, the MPMDSet behaves as previously and POTIM is set to 0.5fs for compounds containing hydrogen, 2fs for anything else.
• fix 1: time_step is now also a class attribute, making the set MSONAble.

Both the time steps, as well as the dict conversion are tested in a new unit test for the MPMD set.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

[shyuep]

Thanks