This pull request continues #1032 from yesterday, and is another step towards #1034.
It's mostly cleanup, but there are some API changes:
Remove the old gammapy/spectrum/sherpa_chi2asym.py (wasn't used within Gammapy or tested, was part of the public API and docs, but I don't think anyone except me years ago to debug HAP FitSpectrum used it)
More uniform names: SEDLikelihoodProfile -> FluxPointProfiles, FluxPointsFitter -> FluxPointFitter (to be consistent with FluxPointEstimator).
change get_test_cases so that it runs during test execution (I think, not verified), not test collection as before.
Change compute_flux_points_dnde to FluxPoints.to_sed_type (a public API change, only caller in Gammapy, the Fermi catalog class is adapted) and move _e_ref_lafferty, _dnde_from_flux to static helper methods on the FluxPoints class (those weren't part of the public API, that's just an internal code organisation simplification).
Misc cleanup of flux_points.py and test_flux_points.py.
Remove docs/spectrum/flux_points.rst (contained only broken examples, and I think the same content is already in docs/spectrum/index.rst and notebooks.