Add 3D fit example using gammapy.maps #1381
This PR modifies the example script for 3D fitting
The fit converges but is quite slow and requires some tweaking of the input parameters. Most annoyingly I had to play a lot with the minimum of the
UPDATE: After ee20503 the covariance matrix is used to determine the inital stepsize in the Minuit fit. This solves the problems described above.