Add HGPS catalog from H.E.S.S.

[cdeil]

The H.E.S.S. HGPS catalog is now out: https://www.mpi-hd.mpg.de/hfm/HESS/hgps/
We used gamma-cat to make it and reference gamma-cat.

I plan to add the new measurements to gamma-cat with a script in the near future.

[pdeiml]

You say that gamma-cat was used to create the catalog, well, what exactly do you want to add to gamma-cat then?

[cdeil]

The new measurements. This was discussed and agreed in the last gamma-cat call: https://github.com/gammapy/gamma-cat/tree/master/notes/2018-03-02
It'll be clear when I make the PR.