Implement signal region optimization tools

[sam-may]

Currently, signal region optimization relies on the ttH/FCNC code (using RooFit + combine). This is fine for our purposes now, but it is a bit messy and the code is hard to read + not as flexible as I would like.

A pure-python implementation would be faster and could be made to be more configurable, readable, and user-friendly.

I'd propose to use zfit [1] as a replacement to RooFit and hepstats [2] as a replacement for combine.

I will plan to do this, but anyone else interested should feel free as well.

[1] https://github.com/zfit/zfit
[2] https://github.com/scikit-hep/hepstats

[mhl0116]

I think both are nice packages, just had a quick look at both.
One exercise that worth doing is to do side-band fit with zfit, didn't find the implementation from a quick look.

If not there, may worth adding or contribute.