Add water tracer ratio checks for certain tracer-specific variables #655

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.