hifact: add support for HistFactory

[sbinet]

it would be great to have some ability to achieve the same things than what ROOT's HistFactory or pure-python's hf do:

• https://github.com/root-project/root/tree/master/roofit/histfactory
• https://github.com/diana-hep/pyhf

[sbinet]

some initial code has been committed inside the hifact branch.