Addition of BeautyTo2CharmTomu3h and BeautyTomuCharmTo3h tools to Gen/GenCuts

Addition of the new tools BeautyTomuCharmTo3h and BeautyTo2CharmTomu3h to the Gen/GenCuts package. These are general purpose tools for filtered semileptonic productions, such that more undesirable candidates are vetoed at the generation phase, avoiding running the full simulation.

The tools, their testing, and their performance are outlined in full in the following SL WG talk: https://indico.cern.ch/event/626598/contributions/2535398/attachments/1436001/2208239/GenCuts.pdf