+Add opt args uhbt_aux and ddu to continuity_PPM

  Added the new optional arguments uhbt_aux, ddu, vhbt_aux and ddv to
continuity_PPM and zonal_mass_flux or meridional_mass_flux to calculate the
differences in barotropic velocity increments for some auxiliary summed
barotropic tranports compared with those used with uhbt or vhbt.  Also cleaned
up and simplified the logic of some of the flags used to apply specified open
boundary conditions, adding a new 1-d logical array thereby avoiding working
unnecessarily on some loops or repeatedly checking for specified open boundary
condition points.  Two openMP directives were also simplified.  All answers are
bitwise identical, but there are new optional arguments to 3 publicly visible
routines.