Fixes #12.

Tito Dal Canton authored Apr 24, 2019 and Leo P. Singer committed Apr 24, 2019

During injection campaigns, search pipelines can create coincs unrelated to
injections.  This adds an option to ligo-skymap-stats to ignore FITS files for
which the inspinj database does not provide a match to an injection.

A misspelled identifier (`parser` instead of `_parser`) caused an
exception to be raised instead of a user-friendly error message to
be printed if the posterior samples file lacked a distance column.

Thanks to @salvatore-vitale for reporting this.

We were using exact floating point comparison to map out the faces
and vertices of HEALPix pixels. When a vertex that is shared
between two faces had slightly different floating-point coordinate
representations in different faces, contours were lost.

In the new approach, we identify vertices by the set of faces that
share them.

Also, the `ligo-skymap-contour` tool got 10x faster thanks to an
optimization that allows us to skip most pixels that are on the
interior or exterior of the contour.