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 level0:lal #29976

00:03:10

 
  Level1
passed level1:lalframe #29977

00:02:01

passed level1:lalmetaio #29978

00:01:41

passed level1:lalsimulation #29979

00:04:00

passed level1:lalxml #29980

00:01:37

 
  Level2
passed level2:lalburst #29981

00:02:07

passed level2:lalinspiral #29982

00:04:23

passed level2:lalpulsar #29983

00:02:54

passed level2:lalstochastic #29984

00:01:54

 
  Level3
passed level3:laldetchar #29985

00:02:22

passed level3:lalinference #29986

00:07:20

 
  Level4
passed level4:lalapps #29987

00:06:42