Add water tracer ratio checks for certain tracer-specific variables

There is at least one new field (`roce`) introduced in PR #638 that is specifically for water tracers, and thus has no bulk water equivalent. However, during a constant ratio test the value of `roce` should match that specified ratio everywhere. So it would be good to eventually add that check to the new `med_methods_FB_check_wtracers` subroutine.

Also, although `roce` is the main variable being considered here, there could certainly be future tracer-only variables in the future, so the checking algorithm should eventually be made as general or extendable as possible. That being said, this request is mostly a "nice-to-have", and is not critical for water tracer implementation in CMEPS or CESM.