+*Non-Boussinesq wave_speed calculations

  Modified the internal wave speed calculation to work in non-Boussinesq mode
without any dependencies on the Boussinesq reference density (RHO_0).  Many
factors of GV%H_to_Z or its inverse are cancelled out in MOM_wave_speed.F90 by
working directly in thickness units.  The code now uses specific volume
derivatives to set the values of g_prime at interfaces when in non-Boussinesq
mode, regardless of whether an equation of state is used.

  This commit also modifies the wave structure calculations in wave_speeds,
which includes the use of thickness_to_dz, changes to the units of three
arguments to wave_speeds and their counterparts in int_tide_CS, and a number of
duplicated calculations of vertical extents mirroring the calculations of
thicknesses.  Some diagnostic conversion factors were modified accordingly in
MOM_internal_tides.

  This commit involves changing the units of 19 internal variables in wave_speed
and 17 internal variables in wave_speeds to use thickness units or other related
units.  There are 6 new or renamed internal variables in wave_speed and 10 new
or renamed variables in wave_speeds.  A total 34 thickness rescaling factors or
references to GV%Rho0 were cancelled out or replaced.  Missing comments
describing the units of several real variables were also added.

  All answers are bitwise identical in Boussinesq mode, but answers do change in
non-Boussinesq solutions that depend on the internal wave speed.  This commit
eliminates the dependencies of the non-Boussinesq wave speed calculations on the
Boussinesq reference density.