Should `ModinDataframe` be a parent of `PandasDataframe`?

[Anatoly Myachev]

ModinDataframe looks almost unused.

[Iaroslav Igoshev]

I think so since ModinDataframe declares the df algebra and PandasDataframe implements it.

[Jonathan Shi]

Seems like this was previously attempted back in #3998, hard to tell at a glance why it didn't end up happening though.