Assign and map a CRS in one step

[benbovy]

This might be a question out of the scope of the PR, but I'm wondering about the need to raise a warning or something if assign_crs is used without subsequently calling map_crs to avoid getting into logically inconsistent situations. Maybe link map_crs in the assign_crs API docs too?

For example, you can assign_crs without warning or error, but then as soon as you try to access one of the .crs values you hit:

ValueError: found multiple CRS in Dataset or DataArray:
spatial_ref: EPSG:32610
lat: EPSG:4326

Originally posted by @scottyhq in #27 (comment)

[benbovy]

(copied from the same comment thread)

Maybe we could allow something like this:

temp = ds_geo_wgs84.proj.assign_crs(
    spatial_ref=("epsg:4322", ["lat", "lon"]),
    allow_override=True
)

I.e., both the new CRS and the coordinates to map that CRS are passed together as a 2-length tuple to .proj.assign_crs(). Note that pyproj.CRS.from_user_input() may also accept a 2-length tuple in the form (‘epsg’, ‘4322’) but that's probably not a big deal.

We could also try to track the CRSes in a Dataset that have been mapped from spatial reference coordinates to other coordinates (or even data variables #21) at the xproj accessor level, but that's tricky to do it right in a lossless way across Xarray operations.