+Add optional argument du_cor to continuity_PPM

  Added the new optional arguments du_cor and dv_cor to continuity_PPM and
zonal_mass_flux or meridional_mass_flux to return the barotropic velocity
increments that make the summed barotropic transports match 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 three
publicly visible routines.