Add water isotope fields to data models

It looks like there are some water isotope fields in DATM, though I'm not sure if they're correct or complete. We need to make sure these fields are correct in the DATM code and then add the necessary water isotope fields in other data models.

This can be guided by the water isotope coupling fields in CMEPS - see https://github.com/ESCOMP/CMEPS/issues/583.

For the initial implementation, these fields should be set to artificial values, such as some fixed fraction of the corresponding bulk water fields. We will probably keep these artificial settings as an option long-term, alongside one or more realistic water isotope forcing datasets.