Michael Mayer / ctools

According to a study by Stephan Fegan done in the context of Fermi/LAT analysis, the best way to determine a butterfly diagram is to draw the envelope of the error ellipse of the model parameters. I implemented this method now in ctbutterfly. I also changed the output file, having now two columns giving as function of energy th minimum and maximum intensities.

Conflicts:
	examples/show_butterfly.py

…(see #1360 for details).

…)-function of individual ctools