make SALT2 model covariance implementation a bit clearer
make SALT2 model covariance implementation a bit clearer

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