Add option to provide photometric observational constraint
radmc3dRunner should get a new function to compute SED at user specified wavelengths (in the future also to compute flux in band instead of single wavelength). \chi^2 is computed from the SED and the supplied observational flux and added to the total model \chi^2 used in the MCMC algorithm. The \chi^2 should be weighted, because otherwise the visibility data would dominate it.