Updates to anndata getting started tutorial

[ivirshup]

Just after I merged 😩

I love this blog post! Awesome that's something like this is finally in the tutorials!

I haven't checked the jupyter notebook, but I think there was something about subsetting .obs by booleans or based on other .obs covariates that had some quirks, no? Also, if you could add sth like .groupby calls on .obs that would be quite nice.

Also, maybe some information on .rename_categories and the effects thereof would be useful (changing stored rank_genes_groups outputs).

Originally posted by @LuckyMD in #10 (comment)

[LuckyMD]

Sorry, I just got around to looking through my emails ^^.