LHCBGAUSS-911: Updates to the EvtFlatQ2 EvtGen model for B to X lepton lepton decays

The EvtFlatQ2 model has been extended to work for any B to X lepton lepton decays to generate a flat q2 distribution (for detector efficiency studies). It also includes a phase space correction to make the distribution flat in the low q2 kinematic region. This will also be suitable for the master branch.