Review and update water isotope coupling fields

[billsacks]

CMEPS has water isotope coupling fields (search for wiso). However, my understanding is that these were just ported over from cpl7 and have never been exercised / tested extensively. For the upcoming water isotope work, we'll need to review and update (if necessary) these water isotope coupling fields.

[billsacks]

We probably will want the capability for runtime-settable, user-defined water tracers - i.e., not just the predefined water isotopes. That could be done as part of this task or deferred to a later task.