Clean up of gammapy-data

[Astro-Kirsty]

We already performed some cleanup through #41.
This proposes further cleanup that needs to be done, post 2.0.

• create a test simulate spectral dataset (see here)
• The data available in tests/unbundled/tev_spectra can be useful. The are presently unused, but it may be useful to document this somewhere (in gammapy.astro maybe). Or, we can remove it...

We should think of a way to properly clean up this folder system. A simple fix for now would be:

• combine all fermi* to under one folder called fermi (same for hawc and ctao)
• we could move the PKS2155-steady to the 'datasets` folder
  • that would result in 7 less folders directly in the main directory
• should decide also on correct naming schemes i.e. using - or _