pop_tools.to_xgcm_grid_dataset does not detect whether the attributes necessary to relabel grid are present

[ALDepp]

I tried to convert a datset with nlat, nlon to the xgcm nlat_t, nlon_t, nlat_u, nlon_u grid using pop_tools.to_xgcm_grid_dataset.

Nothing happened, because I had performed a reduction on the data with xarray before, which removed the attributes, but I also didn't get an error.

It would be nice if pop_tools.to_xgcm_grid_dataset raised an error if there are no attributes, and warning for the variables without attributes in case there are some with and some without.

[mnlevy1981]

@ALDepp -- can you provide a brief example and point out where you would expect an error? Thanks!