Support for GLS / radio projection

• astropy.wcs and G $\backslash$CONVERT are consistent and compatible with the gnomonic absolute coordinates. Note that astropy.wcs claims the projection is SFL, so this means it implicitly translated the reference pixel as requested by CG02.
• ds9 disagree and gives improper coordinates. It seems it acts as if the projection was SFL without translating the reference (which is obviously wrong).