Revised the calculation of gprime and the coordinate densities (GV%Rlay) in
fully non-Boussinesq mode to use the arithmetic mean of adjacent coordinate
densities in the denominator of the expression for g_prime in place of RHO_0.
Also use LIGHTEST_DENSITY in place of RHO_0 to specify the top-level coordinate
density in certain coordinate modes.  Also made corresponding changes to the
fully non-Boussinesq APE calculation when CALCULATE_APE is true, and eliminated
an incorrect calculation of the layer volumes in non-Boussinesq mode using the
Boussinesq reference density that was never actually being used when
CALCULATE_APE is false.

  This commit will change answers in some fully non-Boussinesq calculations, and
an existing runtime parameter is used and logged in some new cases, changing
the MOM_parameter_doc file in those cases.