SED from Cube code

Made a pull request while I work on this in case you have comments.
`GLAT` -> `lat`, `GLON` -> `lon`.

Passing only a mask that defines an arbitrary region would be more flexible, i.e. remote the `lats` and `lons` parameters.
If it doesn't exist yet, you can add a utility function to `gammapy.image` to make a mask for a rectangle in `lon` and `lat`.