L.A.Cosmic: cosmic-ray detection and cleaning

This PR adds the L.A.Cosmic (http://www.astro.yale.edu/dokkum/lacosmic/) algorithm used to identify and clean cosmic rays from single images.
`print` in library code is bad, because it can't be turned off by the user (at least not easily).
I guess there is a standard way to use Astropy logging for such things (if needed at all ... it might be better to optionally return useful info via `return`)?