Make it easy to run just the star selection without fitting a PSF model

[rmjarvis]

Now that we have non-trivial star selection included in Piff, there are use cases where people might just want to run that without doing the subsequent PSF modeling. I think it would be worth adding (1) an output option that is just the initially selected PSF stars from the input file (available whether or not running the PSF model), and (2) a mode that skips the PSF modeling, so this would be the only output.