Support unit-aware arithmetic using pint

[samsrabin]

Proposed new feature or change:

pint is a Python package that allows for unit-aware arithmetic—i.e., automatically harmonizing units as needed during arithmetic operations. It has been mostly integrated into xarray at this point, and I would love to start using it in CUPiD. However, since we support unstructured grids, it wouldn't be smart to bring it in until it's also in uxarray.

Thanks for your consideration!