ENH: Add a `.to_pd_categorical_dtype()` to dataclasses decorated with `@categorical`

[hmgaudecker]

We currently do this in simulation.result:

def _codes_to_categorical(
    *,
    codes: pd.Series,
    categories: tuple[str, ...],
) -> pd.Categorical | pd.Series:

but users may well want to check their own data against this. So it would be neat to have a pd.CategoricalDtype returned from something like:

@categorical
class WorkingStatus:
    retired: int
    working: int

WorkingStatus.to_categorical_dtype()

Added benefit would be that we could use it on any data even if it might not have all outcomes and the dtype will always be correct.