Compute bounds for curvilinear grids

[aulemahal]

Generic Issue

Conservative methods of xESMF (and thus the SpatialAverager) need the longitude and latitude bounds to compute. This is trivial to compute on rectilinear grids: cf_xarray does it for xESMF automatically.

It is non-trivial for curvilinear grids, but possible as such a grid is the projection of a rectilinear grid (at least I think this is the definition of "curvilinear"...). Thus, it is doable by trivially generating the bounds from the original rectilinear grid and then projecting them to a PlateCarree reference system.

I once did this, using cartopy to parse the CF grid mappings into a pyproj object. I'm not sure if cf_xarray would be ok with adding the code there since it would mean having these two package as dependencies (and they aren't light). However, I think we could implement something here first and move it upstream someday.

Part of my old solution lies here : https://gist.github.com/aulemahal/4873db65992369420a96834026b33470

[aulemahal]

There might be useful stuff from Seb and Blaise here (private repo):
https://github.com/Ouranosinc/pysac/blob/master/pysac/affichage/utils.py
https://github.com/Ouranosinc/pysac/blob/8e9c6c5e31c283cceadeba35b3c0cf6b572dc7a5/pysac/utils/grille/interpole.py#L15

[aulemahal]

Finally simple interpolation seems to do the job quite well : xarray-contrib/cf-xarray#370