generating lenses from prior PDFs

Marshall and I wrote a piece of code that draws sets of four point-source positions from a prior over lensing configurations, without explicitly solving the lens equation. This code penalizes configurations by their non-compactness on the source plane, transformed to image-plane units. It works: When we MCMC the prior with Foreman-Mackey's awesome ensemble sampler we get realistic-looking lenses, even though we never actually did any root-finding. The goal is to plug this prior into the Tractor when it is working on images that contain lenses.