Find out translation from real frd to `frd_sigma` parameter 2d psf_model

At the moment `frd_sigma` parameter in the 2d psf_model takes values which are not connected with actual values for the frd (focal ratio degradation). The values that seem to fit the data are around 0.05 to 0.1, but it is not clear what values of frd these numbers actually are. This is because I never made sure that `frd_sigma` corresponds to some realistic values of frd. The task here it to figure how how are values of `frd_sigma` connected with actual values of frd.