• Improve handling of phase-space generation for cases of decays having e+e- pair to make RareLbToLll model useable.
• Some code modernization of RareLbToLll to make things const where they are constant and pass const references instead of raw pointers in most places.

Tested locally that nothing changes for Lz mu mu final state. In case of Lb --> Lz e+ e-, without this change model is effectively unuseable as maximum probability gets too high and accept-reject fails to find working configuration in 1000 trials. After change code can generate 1M events without any issue and gives expected q2 distribution. I'm not adding test as I will do this directly to new framework.