Add methods to calculate sky map overlap with multi-ordered sky maps; fixes #19

CHANGES.rst

bin/raven_calc_signif_gracedb

bin/raven_coinc_far

bin/raven_skymap_overlap

ligo/raven/gracedb_events.py

ligo/raven/search.py

ligo/raven/tests/FAR_study.py

ligo/raven/tests/FAR_subgrb_study.py

ligo/raven/tests/data/GW170817/GW170817.fits.gz

ligo/raven/tests/data/GW170817/GW170817.multiorder.fits

...ests/data/GW170817/glg_healpix_all_bn_v00.multiorder.fits

ligo/raven/tests/data/basic_skymaps/bayestar.multiorder.fits

.../raven/tests/data/basic_skymaps/fermi_test_skymap.fits.gz

ligo/raven/tests/data/basic_skymaps/flat-skymap.fits.gz

...aven/tests/data/basic_skymaps/flat-skymap.multiorder.fits

.../raven/tests/data/basic_skymaps/swift_test_skymap.fits.gz

ligo/raven/tests/test_overlap_integral.py

ligo/raven/tests/test_raven_search.py