Example bump

This is still a work in progress, but I need some input, so I am using a pull request to discuss the code.
I do not understand how to use the new setup: I thought that the 'model' and template were not needed anymore, but it complains with an `AssertionError: A template is required`.
Do you have a working example?

Also, I have two comments/suggestions for `pyFBU`:
- we should specify the input *data*, not the json file (the input can be either from json or from another source, we don't care)
- one of the input parameters is the bkg. This is not always well defined (the bump case is one example where we don't need to differentiate between bkg spectrum and signal spectrum, b/c we're unfolding one spectrum). I think we should have by default a null background, i.e. `nreco*[0.]` and `ntruth*[0.]`
Please let me know what you think.

Thanks

Davide