[WIP] Proof of concept for not converting objects to strings

[ivirshup]

Potentially fixes #115. This just preserves objects during strings_to_categoricals.

This will change behaviour for cases where there we mixed types in a Series (like: ["abc", 0.6]), but I'm not sure if that's a common case. It will still convert if there are missing values.

This still converts things to strings for writing, where it should probably error or at least warn.

[LuckyMD]

I remember having issues when having object as dtype in obs. I'm not sure what it was... but I remember there was a long back and forth with me being clueless as to why sth didn't work. Not very helpful atm... but just a heads up.