Fitter
======

The fitter block is unique in that it is a class not implemented as part of
`photutils`.  Rather, it is an object that follows the iterface for fitters in
the `astropy.modeling` package.   Note that implicitly the fitters for
PSF photometry are always fitters appropriate for *2D* models (i.e., 2 input
dimensions for the x and y pixel coordinates and a "flux" output).
