+Add summed_continuity & alter zonal_mass_flux

  Added a new subroutine, summed_continuity, to update the total ice and snow
mass fluxes and update the total ice, snow, and pond mass without
differentiating between thickness categories, and possibly also to update the
ice thickness.  Also changed two of the arguments to zonal_mass_flux and
meridional_mass_flux into optional arguments, reordered the list of arguments,
and added two new optional arguments to these routines to work on summed fluxes
or fluxes by thickness categories.  All answers are bitwise identical, but there
are new or altered interfaces.