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.
Probably this is at least part of the issues, no?
The checks / handling for this input coordinate system isn't that simple, so it gets set up incorrectly for some input files.