assign "spatial ref" variable attrs

[dcherian]

diffcrs.proj.assign_crs(spatial_ref="epsg:4326")

for a dataset without an existing spatial_ref, this will create a new spatial_ref variable with no attrs. This is confusing because if the spatial_ref variable was present, those existing attrs will be preserved (and possibly out-of-date).

I think it would be good to be consistent here --- either the spatial ref always has attributes (kept in sync with the index) or it never has attributes.

[benbovy]

Agreed. It is possible to always have attributes kept in sync with the index (by implementing CRSIndex.create_variables()). However, we would need to choose and stick with one set of conventions for the attributes, and we would need to handle that in .proj.write_crs_info().

We should also check for conflicts between the CRS passed as an option and any existing attribute of an existing spatial_ref (I don't think we're doing it currently).

[dcherian]

Another confusing one is when there is no spatial_ref variable, it is created with no attributes.

ds = xr.tutorial.open_dataset("ersstv5")
ds.proj.assign_crs(spatial_ref="epsg:4326")