PSF fitting

Fadely, Foreman-Mackey, and I worked on some point-spread-function fitting in preparation for making adjustments to how this is done in The Thresher and perhaps also to criticize how it is often done in standard pipelines. There are many issues: What basis to use for the PSF, what weighting to use in the fits, and what regularizations or priors to apply. The point we are coming to is that if you apply the non-negative regularization, you have to also put on other regularizations or else you can get highly biased results.