Enabling bin-moving events within the Propagator engine

[nadrino]

Some parameters like energy scale can be defined as a distorsion of event observables.

This means at each evaluation of the Propagator, some event would request a new bin search wrt sample or parameter dials.

In current statistical analysis, these parameters are treated as an effective reweight factor that simulate those migrations. This technique has historically been chosen for improving performances and insuring the continuity of the second derivative of LLH. One of the use is the lack of accuracy for propagating the systematics since this effective method uses an a priori shape of the involved spectra.

Having this feature would allow the user to have a more accurate propagation of systematics or allow to build the appropriate reweight dials to be used in a given analysis.