Add polygon query

[cdeil]

A big feature astropy.healpix that we're still missing is polygon query.

The algorithm used in the C++ lib for HEALPix polygon query (and also disk query) is outlined in http://adsabs.harvard.edu/abs/2015A&A...580A.132R . In healpy it's exposed like this: healpy.query_polygon

For both polygon and disk query, efficient options to return the result is range sets (see the paper mentioned above), or MOCs (see https://github.com/cds-astro/mocpy) (cc @tboch). Another question is how to avoid duplication with astropy-regions which already has a polygon class with contains implemented (although at the moment only for cartesian, not spherical polygons, but that should be added there).

In this package I see that @dstndstn put a point_in_polygon, but I don't think HEALPix query_polygon exists, only disk query. @dstndstn - Is that correct, or do you have code for this somewhere already?

@tboch also mentioned that some use cases require to know whether a given pixel is entirely or partially covered is needed; info that currently isn't available by our cone_search API.