This section looks like a fine *advanced* example, but what about the most simple thing.  Assume that `astropy.modeling` has some built-in statistics: `chi2` (or `leastsq`), `cstat`, `cash`, `chi2dvar` (variance estimated from data), `chi2mvar` (variance estimated from model).  How would a user switch between the different statistics and re-run their fit, regardless of the exact model and optimizer?