Add posterior samples to FITS sky map postprocessing

This moves development from the original repository:
https://github.com/farr/skyarea

Some minor changes occurred in the course of refactoring:
 * The contents of the module `sky_area.sky_area_clustering` moved
   to `lalinference.kde`.
 * The Python script `run_sky_area` was renamed to
   `lalinference_samples_to_fits` and the command line interface
   was ported from `optparse` to `argparse`.
12 jobs for skyarea in 25 minutes and 37 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Level0
passed #29976
level0:lal

00:03:10

 
  Level1
passed #29977
level1:lalframe

00:02:01

passed #29978
level1:lalmetaio

00:01:41

passed #29979
level1:lalsimulation

00:04:00

passed #29980
level1:lalxml

00:01:37

 
  Level2
passed #29981
level2:lalburst

00:02:07

passed #29982
level2:lalinspiral

00:04:23

passed #29983
level2:lalpulsar

00:02:54

passed #29984
level2:lalstochastic

00:01:54

 
  Level3
passed #29985
level3:laldetchar

00:02:22

passed #29986
level3:lalinference

00:07:20

 
  Level4
passed #29987
level4:lalapps

00:06:42