`WCS.pixel_to_world` does not have the same broadcasting behaviour as `astropy.wcs.WCS`

[Cadair]

I have to do this:

    coords_wcs = wcs.pixel_to_world(np.arange(2560), 0, 0)
    coords_gwcs = gwcs.pixel_to_world(*np.broadcast_arrays(np.arange(2560), 0, 0))