Add a tutorial with UL computation

[cosimoNigro]

I think it would be nice to have a notebook showing how to compute UL on a weak or non-detected source.
This is a science case as much as estimating a spectrum or a light curve.

In particular, the tutorial could show how to compute ULs for different confidence levels, or how to change the threshold statistics to be used to define them.

[registerrier]

Thanks @cosimoNigro . This is a good idea. Could this be part of a source detection tutorial? e.g. at the end of this tutorial?

What would you show in a stand alone tutorial?

• how to use the Fit.confidence and Fit.stat_profile to explore the amplitude parameter UL?
• or use a FluxEstimator to do so?

[adonath]

Note that the MapEstimator also compute ULs, so for simple cases one would just read it off the UL map.