Fully support tweak_latlons.py

I wrote [tweak_latlons.py](https://github.com/ESCOMP/CTSM/blob/master/tools/contrib/tweak_latlons.py) to avoid [answer differences due to processor count changes](https://github.com/esmf-org/esmf/issues/276#issuecomment-2817200350). At the time, we had hoped this would be temporary stopgap, so I put it in `tools/contrib/`. Unfortunately, it looks like the ESMF fix won't come in until after CESM3/CTSM6. Because we will need to use such "tweaked" files in the release, we should turn it into a fully-supported (and tested!) script under `python/ctsm/`.