Review old water_isotope code; remove or rework it as needed

[billsacks]

There is carry-over water isotope code from the CESM1 water isotope implementation:

https://github.com/ESCOMP/CESM_share/tree/main/src/water_isotopes

We should review this code and decide what to keep, what to remove, and what to rework.

If we keep these modules, we should give them a shr_ prefix for consistency.