add attributes for generated x,y variables

[dcherian]

We could at least do standard_name and units. I believe https://pyproj4.github.io/pyproj/stable/api/crs/coordinate_system.html#pyproj.crs.CoordinateSystem.to_cf does what we want.

[dcherian]

#45 adds this by default but it's a bit of a hammer.

We could consider adding another xproj hook: #45 (comment)

Also not nice is that assign_index will first drop any existing coordinate variables, and then regenerate them with these attributes. We should probably allow some way to preserve any existing attributes.

For now I will merge #45 because I am very partial to well annotated datasets ;) and we can adapt later