SED from Cube code

Made a pull request while I work on this in case you have comments.
I think the order is incorrect here ... according to the docs `GammaSpectralCube.spatial_coordinate_images` returns `lon, lat` and not `lat, lon` as you write here.
See https://gammapy.readthedocs.org/en/latest/api/gammapy.spectral_cube.GammaSpectralCube.html?#gammapy.spectral_cube.GammaSpectralCube.spatial_coordinate_images