Add water tracers

[billsacks]

Description of changes

Core changes needed to add water tracers in CESM, for the sake of water isotopes and other purposes.

Major pieces of this include:

• XML and config variables for defining water tracers
• Addition of all necessary water tracers in esmFldsExchange_cesm_mod and fd_cesm.yaml. These use an ungridded dimension for water tracers.
• Calls to a routine to check that test tracers maintain their initial ratios. This ensures that tracers are being treated the same as their bulk counterparts (in terms of mapping, merging, etc.).
• Addition of water tracer fields in med_test_comps, so this tracer infrastructure can be verified in an X compset test
• Various other changes to some of the med_test_comps to better exercise CMEPS functionality, especially for the lnd -> glc mapping; also added some fields from GLC and to ROF in med_test_comps: Fgrg_rofi and Fgrg_rofl

Some pieces are not yet implemented:

• Including water tracers in the custom lnd->rof mapping
• Including water tracers/isotopes in atm-ocn flux calculations
• Including water tracers in the budget tables (med_diag_mod)

Depends on ESCOMP/CESM_share#82

Specific notes

Contributors other than yourself, if any:

CMEPS Issues Fixed (include github issue #):

• Resolves Review and update water isotope coupling fields #583 (though this turned out to be much more extensive than simply reviewing and updating what had been in place before)

Are changes expected to change answers? (specify if bfb, different at roundoff, more substantial) Changes answers for X compsets (due to some changed counter logic and maybe other things); should be bit-for-bit for all other compsets

Any User Interface Changes (namelist or namelist defaults changes)? Adds xml and config variables controlling water tracers

Testing performed

Only minimal testing so far. I will test this more extensively and update this when I do.

[billsacks]

I still want to do a more extensive review and testing of this myself, so for now marking this as draft.

[billsacks]

@nusbaume I have tentatively assigned you as primary reviewer, but I could reassign to someone else.

@mvertens and @Katetc - also looping you in

[billsacks]

I'm putting the new variables in ALLCOMP_attributes. I'm not sure if this is right, or if these should go in DRIVER_attributes. I chose ALLCOMP_attributes just because there was more stuff (and more relevant stuff) there, but I'm not sure why I'd choose one over the other.

But actually, maybe ALLCOMP_attributes is needed given that MOM may need to access these rather than going through the share code??

@mvertens do you have guidance here?

[mvertens]

@billsacks - if any component needs this variable - then it should belong in ALLCOMP_attributes. I think I envisioned the DRIVER_attributes as having variables that are available on all processors but are specific to the driver itself (e.g. pio, etc).
I am not sure why the following variables

     reprosum_diffmax
     reprosum_recompute, 
     reprosum_use_ddpdd
     wv_sat_scheme
     wv_sat_table_spacing
     wv_sat_transition_start
     wv_sat_use_tables

are in the DRIVER_attributes and not in the ALLCOMP_attributes.
@jedwards4b - what are your thoughts on this?