Replace Pandas with Narwhals in Statespace #575
Description
The PyMCStatespace class does more data preprocessing than just PyMC, for example here. Right now, all the logic is pandas specific, and we even care about the type of the index (!!). It would be good to make this more agnostic, so that users can bring data using whatever dataframe library they like.
Narwhals is a package that is meant to make it easy for packages to be agnostic to what is being passed in, so it seems like a good fit for us here.
Related to pymc-devs/pymc#7462