*+non-Boussinesq revisions to MOM_generic_tracer

  Revised initialize_MOM_generic_tracer to use thickness_to_dz to get the layer
vertical extents and then provide these to MOM_initialize_tracer_from_Z to read
in initial generic tracer concentrations from Z-space files.  The previous
approach inappropriately used an simple multiplicative rescaling (via a call to
dz_to_thickness_simple in MOM_initialize_tracer_from_Z) by a factor that
includes the Boussinesq reference density when in non-Boussinesq mode.  A new
thermo_vars_type arguments was added to initialize_MOM_generic_tracer to allow
for this change.

  Also revised MOM_generic_tracer_column_physics to use thickness_to_dz instead
of a simple multiplicative rescaling to get the layer vertical extents (in m)
that are used in calls to generic_tracer_source.  The multiplicative factor that
was used previously (GV%H_to_m) includes the Boussinesq reference density and
hence is inappropriate in non-Boussinesq mode; using thickness_to_dz avoids
this.

  Also added comments documenting the meaning and units of about 30 real
variables in the MOM_generic_tracer routines.

  There is a new mandatory argument to initialize_MOM_generic_tracer.  All
Boussinseq mode answers are bitwise identical, but in non-Boussinesq mode mode
generic tracer answers are changed by avoiding the use of the Boussinesq
reference density in several places.