Support item assignment for `Grid`

[philipc2]

Pointed out by @brianpm

I'm having some trouble because that grid file is in radians, so the uxds.uxgrid Spherical Coordinates are in radians. I tried to convert them in place, uxds.uxgrid['node_lon'] = np.rad2deg(uxds.uxgrid['node_lon']), but that doesn't seem allowed (TypeError: 'Grid' object does not support item assignment).

uxds.uxgrid['var'] = ... should operate the same as uxds.uxgrid.var = ...