Update climatology entries to reflect Xarray GroupBy updates

[kafitzgerald]

Deepak gave a great Pangeo Showcase the other week on updates to Xarray's GroupBy. It'd be great to incorporate some of this here e.g. in the climatology entries.

Slides and notebooks here: https://gist.github.com/dcherian/42411d8645573245045f0a55febf5fbe

[kafitzgerald]

So I'm not exactly sure we want to add the additional dependencies required for this (at least w/ flox), but I think adding a note in the relevant sections with some links to docs / tutorials would be great.

We could also add in some content about the Xarray groupers, but maybe that's an item for later as it's needed and/or more clear where it might fit in.

I'll try to add in those links mentioned above in the next day or two.