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.