Add HEALPIX reprojection

Tool to convert HEALPIX images to Galactic or ICRS coordinates, based tightly on @cdeil's function in gammapy.  See https://github.com/healpy/healpy/issues/129.
There is now a function in astropy that does this correctly:

https://github.com/astropy/astropy/blob/master/astropy/wcs/utils.py#L114