Refactoring density integrals for efficiency

  Refactored 4 routines (int_density_generic_pcm, int_density_generic_ppm,
int_spec_vol_generic_pcm and int_spec_vol_generic_plm) in density integrals for
greater computational efficiency by doing fewer calls to the equation of state
routines but calculating entire rows of densities at subgrid locations with each
each call, replicating what was already being done int_density_dz_generic_plm.
To accomplish this, a number of variables now use larger arrays than
previously.  The total computational cost of the non-Boussinesq pressure
gradient force calculation was more than 50% greater with the previous code in
some tests.  All answers are bitwise identical.