+Rescaled variables in MOM_neutral_diffusion.F90

  Rescaled pressure and density variables in MOM_neutral_diffusion and added
numerous comments describing internal variables and their units.  Some unused
variables were deleted, including unused pressure arguments to
find_neutral_pos_linear.  In addition unit_scale_type arguments were added to 8
subroutines, including neutral_diffusion_init and tracer_hor_diff_init.  All
answers are bitwise identical, but there are changes to public interfaces.