Document MOM_regridding and coord variable units

  Added or amended comments describing about 90 variables in the MOM_regridding,
coord_adapt, coord_rho, and coord_sigma modules to document the purpose and
units of real variables using the MOM6 standard syntax.  A missing doxygen
comment was also added to describe inflate_vanished_layers_old().  A few unused
internal variables were also eliminated, including the max_depth_index_scale
element of the regridding_CS type.  All answers are bitwise identical and only
comments are changed.