Determine whether data is divergent and use a RdBu colormap / center clim

[ahuang11]

#205

The centering is based off all time step / all dimensions.

import hvplot.xarray
import xarray as xr

ds = xr.tutorial.open_dataset('air_temperature')
da = ds['air']
da_anom = da - da.mean()
da_anom.hvplot('lon', 'lat')

[jsignell]

Ok I think this is reasonable now.

[jsignell]

Well I guess it's not quite done. Still need some more tests around non-numeric datatypes I suppose. Or put back in the try/except.

[jsignell]

@philippjfr good with you if I merge?