Helper function to use adata.uns[obs_set_col + "_colors"] for initial obs set colors

[keller-mark]

As suggested in #339

As a ease of use functionality, It would be handy if colors could be manually set with the adata.uns[f"{obs_category_column}_colors"] which is a list of RGB values sorted by adata.obs[obs_category_column].categories.cat. Many scanpy functions take into account these manual color settings as well when plotting.