Add testing and documentation for tweak_latlons.py

I made the tweak_latlons.py script to work around the issue of processor count changing nearest-neighbor planting date (issue esmf-org/esmf#276). I put it in tools/contrib/ and didn't add any testing because a fix for that bug had been planned for CESM3, so the script would no longer be needed. However, that's no longer the plan, so I'll need to make this an "official" script in python/ctsm/, with all the testing and documentation that entails.