FutureWarning in apply_ufunc

[slevang]

Since pydata/xarray#4060, xesmf has given a FutureWarning for supplying output_sizes directly in xr.apply_ufunc rather than through dask_gufunc_kwargs.

import xarray as xr
import xesmf as xe

ds = xr.tutorial.open_dataset('air_temperature')
regridder = xe.Regridder(ds, ds[['lon', 'lat']], 'bilinear')
regridder(ds)

xesmf/frontend.py:555: FutureWarning: ``output_sizes`` should be given in the ``dask_gufunc_kwargs`` parameter. It will be removed as direct parameter in a future version.
  ds_out = xr.apply_ufunc(

This eliminates the warning.

[raphaeldussin]

@slevang thank you for this fix, it LGTM
@huard I'll let you merge it in

[huard]

Back from vacations.
Do we need stricter limits on supported xarray versions with this ?

[slevang]

You're right, looks like this kwarg was added to xarray in 0.16.1 (pydata/xarray#4060), which had other issues as well and needed to be avoided. So we need xarray>=0.16.2 I believe.