SED from Cube code

Made a pull request while I work on this in case you have comments.
I find it more readable if you do
```
table = Table()
table['ENERGY'] = energy
table['DIFF_FLUX'] = Quantity(values, cube.data.unit)
table['DIFF_FLUX_ERR'] = Quantity(errors * values, cube.data.unit)
```
It's one simple statement per line instead of a complex statement across four lines,
and it has column name and value together.