Introduce Fake Factors method

MR Description

This MR introduces the Fake Factor method for the estimation of jet->τh fakes. The full description of the method is given in this MR and in this presentation.

The FFs method is introduced in parallel to the already available ABCD method. In this way any of the two can be chosen at will.

From the NBA point of view, the FFs method can easily be called with the options --PrePlot-do-ff True --do-ff True, provided that in the analysis code specific to each analysis the FFs are computed as described in the linked MRs and presentation.

MR Validation

Validated by computing FFs for one period, applying them, and comparing the result to the standard ABCD method.