Cancelled out factors of GV%Z_to_H in MOM_hor_visc.F90 to simplify the code
and reduce the dependence on the value of GV%Rho_0 in non-Boussinesq mode.  This
involved changing the units of 3 internal variables in horizontal_viscosity and
one element in the hor_visc_CS type to use thickness units or their inverse.
Because GV%Z_to_H is an exact power of 2 in Boussinesq mode, all answers are
bitwise identical in that mode, but in non-Boussinesq mode this conversion
involves multiplication and division by GV%Rho_0, so while all answers are
mathematically equivalent, this change does change answers at roundoff in
non-Boussinesq mode.