cleanup obsolete options and parameters

Details:
- delete now-obsolete options and parameters (model, template,...)
- reduce vertical space
- update docstrings
remove empty lines, remove default parameters

Details:
these parameters are required. Without them we cannot run any fbu, and
we should just exit. We should specify default values only once, in
the top-level script, as cmd-line defaults.
update default values
simplify and clarify loop

Details:
prefer list comprehension where possible.
This also makes clear that we're adding a bkg contribution and a
migration contribution.
It also allows for an empty background (will be needed for the bump,
for example).
fix a bug in the unfolding method, and add back the model name to save the summary histogram with a different name
remove the printout
Merge pull request #34 from gerbaudo/cleanup-defaults

Cleanup defaults, good to merge