Simplify the return type of `IncrSmallStrainModel.history_dim()`

My thought process behind the return type

def history_dim(self) -> int | dict[str, int | tuple[int, int]] | None:
        ...

was that

int is for the case where we don't have the possibility of having named history in a dictionary because we use for example an ABAQUS UMAT which uses a vector of lenght $n_\mathrm{hist}$
dict[str, int | tuple[int, int]] is for the models that we implement in Python or Rust or C++, where having a dictionary is not a problem and it is just way more convenient to have a separate function for each history variable.
None: there is no history

Now, I noticed that handling 3 cases each time something is done to the history, produces a lot of ugly boilerplate code, so I would propose

def history_dim(self) -> dict[str, int | tuple[int, int]] | None:
        ...

and case 1. can be handled by sth. like {"umat_history": n_hist}.

@pdiercks , @aradermacher , @saifr68 , what do you think?

BAMresearch / fenics-constitutive