Proof of concept - public Grouper objects

[dcherian]

Not for merging, just proof that it can be done nicely :)

Now builds on #8840
Builds on an older version of #8507

Try it out!

importxarrayasxrfromxarray.core.groupersimportSeasonGrouper, SeasonResamplerds=xr.tutorial.open_dataset("air_temperature")
# custom seasons! ds.air.groupby(time=SeasonGrouper(["JF", "MAM", "JJAS", "OND"])).mean()
ds.air.resample(time=SeasonResampler(["DJF", "MAM", "JJAS", "ON"])).count()