new API for CompositeMagSystem: add families
new API for CompositeMagSystem: add families

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