Clean up pass_var for tracer reservoirs.

[kshedstrom]

Use new options to pass_var, saves having to allocate both directions of tracer reservoirs if only one is used.

[codecov-commenter]

Codecov Report

Merging #1205 into dev/gfdl will decrease coverage by 0.01%.
The diff coverage is 37.95%.

@@             Coverage Diff              @@
##           dev/gfdl    #1205      +/-   ##
============================================
- Coverage     46.08%   46.06%   -0.02%     
============================================
  Files           214      224      +10     
  Lines         69399    70581    +1182     
============================================
+ Hits          31984    32516     +532     
- Misses        37415    38065     +650     

Impacted Files	Coverage Δ
...g_src/external/GFDL_ocean_BGC/FMS_coupler_util.F90	0.00% <0.00%> (ø)
...fig_src/external/GFDL_ocean_BGC/generic_tracer.F90	0.00% <0.00%> (ø)
...c/external/GFDL_ocean_BGC/generic_tracer_utils.F90	0.00% <0.00%> (ø)
config_src/external/ODA_hooks/kdtree.f90	0.00% <0.00%> (ø)
config_src/external/ODA_hooks/ocean_da_core.F90	0.00% <0.00%> (ø)
config_src/external/ODA_hooks/ocean_da_types.F90	0.00% <0.00%> (ø)
config_src/external/ODA_hooks/write_ocean_obs.F90	0.00% <0.00%> (ø)
config_src/solo_driver/MESO_surface_forcing.F90	0.00% <0.00%> (ø)
config_src/solo_driver/user_surface_forcing.F90	0.00% <0.00%> (ø)
src/ALE/MOM_regridding.F90	31.47% <0.00%> (-0.17%)	⬇️
... and 201 more

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[Hallberg-NOAA]

This PR is being tested with https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/11164.

[Hallberg-NOAA]

These changes are correctly exploiting recently added capabilities to reduce the need for excessive memory allocations and message passing.

This commit pull request should be acted upon as soon as the pipeline tests have passed.