Batch by any (categorical) column

[hombit]

Currently, we can batch sub-light curves with batch(by_band=True). It is a great feature, but I believe we would benefit from some refactoring of it.

Me feeling is that current batch interface is over-loaded and has too many combinations of 1) analysis function type, 2) its Dask meta types, 3) grouping-by flavors and 4) types of the output.

I propose to introduce batch_by(by=column_name) and its thin wrapper batch_by_band to replace the current batch(by_band=True) interface. It would simplify our interfaces, make the code more maintainable, and allow users to use any categorical column, not band only.