Let process_crs try a few approaches before raising error

[jsignell]

This PR makes the process_crs more generous in what it accepts and adds tests for the function.

[jsignell]

I'm not 100% sure why the python 3.7 jobs are failing, but I would expect this new test to fail on main in all cases, so I think it would be reasonable to skip the new test for python 3.7 and call it a day. It is more likely tied to the pyproj version though. It looks like pyproj dropped support for python 3.7 a while back so that is probably related.

[hoxbro]

I'm fine with skipping the failing test on Python 3.7.

[maximlt]

Thanks @jsignell ! Looks good to me.

Note for myself, crs accepts:

• 'EPSG:<epsg_code>' as str
• 'epsg:<epsg_code>' as str
• <epsg_code> as int
• PROJ.4 string
• pyproj.Proj
• cartopy.crs.CRS
• and probably more :)

Also the utils that parse the projection like proj_to_cartopy and check_crs are from salem, need to check if they've been updated.