SED from Cube code

Made a pull request while I work on this in case you have comments.
Why did you change this?
`compute_differential_flux_points` is listed in `__all__` in `flux_point.py`.

We do not want to have a duplication of the manual `__all__` lists in the `__init__.py` files ... that would be hard to maintain (and there's no reason to do it).