Adding ability to pass `ligo-skymap-from-samples` a `pesummary` metafile

The purpose of this MR is to allow the user to pass a PESummary metafile to the ligo-skymap-from-samples executable. I have had to add an extra command line argument --preferred_run, because you are able to store multiple analyses in a single PESummary metafile and therefore this option allows the user to choose a specific analysis. If nothing is chosen, it will choose the 0th label by default. Fixes issue #13 (closed).