How to make model and residual images and spectra

[cdeil]

The http://docs.gammapy.org/dev/notebooks/analysis_3d.html tutorial needs to be amended, to show how to make model and residual images and spectra.

Basically everyone that did a MapFit will want to (or should) use that to check the result.

I know time is very short, but if someone could extend the notebook today or tomorrow latest a little bit, to mention e.g. that fit.evaluator.compute_npred() will give a model image, that would be very useful.

[cdeil]

Done by @adonath in #1821 .