Use the new runtime parameter RHO_PGF_REF instead of RHO_0 to set the
reference density that is subtracted off from the other densities when
calculating the finite volume pressure gradient forces.  Although the answers
are mathematically equivalent for any value of this parameter, a judicious
choice can reduce the impacts of roundoff errors by about 2 orders of
magnitude.  By default, RHO_PGF_REF is set to RHO_0, and all answers are bitwise
identical.  However, there is a new runtime parameter that appears in many of
the MOM_parameter_doc.all files.