Extended sources

Gets extended sources from fermi catalogs - required for a function to create fits Images from catalog tables (with the option of including extended catalog sources) which will be added in a future pull request.
This will result in a failing doctest once we turn doctesting on for Gammapy.
Maybe simply remove this line?
```
>>> sources[0] = <astropy.io.fits.hdu.image.PrimaryHDU at 0x7f7c86665290>
```