fix Model.bandflux_rcov bugs
fix Model.bandflux_rcov bugs

dannygoldstein/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,dannygoldstein/sncosmo,sncosmo/sncosmo,kbarbary/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,kbarbary/sncosmo