EvtGen model to generate 4-body phase-space decays in restricted m12-m34 window

In the 4-body phase-space decays chance to generate events in the corners of m12-m34 region is rather small and thus selecting relevant events from the full 4-body phase-space is rather inefficient. Additional uses cases where just a small part of the phase-space is needed exists (like one in https://its.cern.ch/jira/browse/LHCBGAUSS-2173). While this model does not fully remove accept-reject, it tries only events, which fall into the desired mass window and also sets maximum probability based on the requested window, just allowing the more efficient generation of events in a small window with a small probability.

Code itself was already reviewed by @jback at https://phab.hepforge.org/D47 and already is on its way to HepForge.