Use RHO_KV_CONVERT instead of RHO_0 to set the non-Boussinesq version of
GV%m_to_H, so that there is a mechanism for testing the independence of the
fully non-Boussinesq mode from the Boussinesq reference density.  With this
change, GV%Z_to_H is not guaranteed to be equal to (GV%Z_to_m*GV%m_to_H), with
the latter expression preferred when setting parameters. By default the two
parameters are the same, and they will probably only ever differ in testing the
code.  All Boussinesq solutions are bitwise identical, but there are differences
in the description of RHO_KV_CONVERT that will appear in MOM_parameter_doc
files.