(*)Avoid using RHO_0 in non-Boussinesq averaging

[Hallberg-NOAA]

Use GV%H_to_MKS instead of GV%H_to_m when undoing the dimensional rescaling of thicknesses when taking weighted averages in horizontally_average_diag_field, global_layer_mean and global_volume_mean. In Boussinesq mode, these are identical, but in non-Boussinesq mode using GV%H_to_m introduced a multiplication and then division by the Boussinesq reference density, whereas GV%H_to_MKS avoids this by rescaling to a volume or mass-based coordinate depending on the mode. Several comments were also updated to reflect these conditional changes in the units of some internal variables. All expressions are mathematically equivalent, and this does not impact any solutions, but there can be changes in the last bits in some non-Boussinesq averaged diagnostics.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (541c2f4) 37.20% compared to head (490feae) 37.20%.

Additional details and impacted files

@@            Coverage Diff            @@
##           dev/gfdl     #547   +/-   ##
=========================================
  Coverage     37.20%   37.20%           
=========================================
  Files           271      271           
  Lines         80353    80355    +2     
  Branches      14985    14985           
=========================================
+ Hits          29894    29897    +3     
  Misses        44902    44902           
+ Partials       5557     5556    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[marshallward]

Approved. Calling it volume is a bit confusing but I understand the intention, and renaming it would be out of scope of this PR.

[marshallward]

Gaea regression: https://gitlab.gfdl.noaa.gov/ogrp/MOM6/-/pipelines/22156 ✔️