Update zero mag fluxes for the new "siriusvega" system

[karllark]

This is an update needed to be consistent with flux calibration for JWST (and HST). Sirius is the color standard and the siriusvega system is that color standard shifted to the "vega" system to keep (as much as possible) consistency with the "vega" system (that has multiple definitions in the literature - hence the need for a new cleaner definition).