Update climatology functions following Xarray GroupBy enhancements

[kafitzgerald]

Deepak gave a great Pangeo Showcase the other week on updates to Xarray's GroupBy.

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

These updates offer improved performance and increased flexibility for things like custom seasons (though don't address things like weighting).

It'd be good to go through our climatology functions to check for compatibility and/or whether or it may make sense to deprecate some of our functions now that upstream functionality has been enhanced.

GeoCAT Applications seems like a good place to begin exploring the latter bit (e.g. in NCAR/geocat-applications#150).