Add option to mesh_modifer tool to output mask as either ocean-mask (as is now) or as a land-mask

It's useful for the mesh_modifier tool to output the mask as either an ocean-mask (so 1 means ocean, and 0 means land) or as a land-mask (so 1 means land, and 0 means ocean). A simple command line option can easily be added to trigger between these two options.