Added check for ascending order error in SALT2Source.bandflux_rcov
Added check for ascending order error
in SALT2Source.bandflux_rcov

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