Flux point method

Implements #50 .
The fluxes in `gammapy/datasets/data/tev_spectra/electron_spectrum_fermi.txt` are differential fluxes, you can't use them for this example ... you should either make your own example starting e.g. with an exponential cutoff power law and computing integral fluxes, or use the Fermi 2FGL catalog which contains integral fluxes.