Healpix version 3.11

See http://healpix.jpl.nasa.gov/

HEALPix is an acronym for Hierarchical Equal Area isoLatitude
Pixelization of a sphere. As suggested in the name, this pixelization
produces a subdivision of a spherical surface in which each pixel
covers the same surface area as every other pixel.  Healpix is used
extensively in astronomy to produce maps of data on the sky.

Closes #169.