Get water isotope forcing fields from the coupler

For now, we're setting the incoming water isotope/tracer forcings (from atm and rof) to some fixed constant times the bulk. Eventually, we want to get the actual values from the coupler.

I'm not sure if anything needs to be done in order for this to work with our current datm forcings - e.g., should datm set the isotope forcings to a constant times the bulk if they don't exist on the dataset? Or should that be the responsibility of CTSM?

We'll probably also want a consistency check that ensures that, if we're running with isotope fields in CTSM, then the necessary coupling fields actually exist. If they don't, then we should either abort or set the isotope fields equal to the constant times the bulk (I'm not sure which is the correct thing to do).