Add systematic errors in the error columns of the SED specifications

[cosimoNigro]

I was playing along with SED flux points and radiative models. Having the systematic error specified for the flux points, beside improving the fit, takes properly into account the inter-calibration effects in MWL SEDs.

What do you think of adding systematic errors in flux among the SED error columns? It can be a new column like {NORM_REP}_sys or {NORM_REP}_syserr. The experiment producing flux points and statistical errors might thus add its own estimate of systematic errors. Or one might extend an already existing SED file adding a column following what an experiment claims (e.g. 15% in flux).

Of course one could also take into account the systematic errors in energy (i.e. along the x-axis) but I think this is more complex and for the time being the ones in flux are a good start.