Correct the beam particle number sampling in beam background simulation
A minos debug in beam background sampling:
Previously:
Nparticle = Poisson((int)rate * time window)
Now:
Nparticle = (int)Poisson(rate * time window).
Fixed the issue that Nparticle is always 0 if (rate * time window)<1
Edited by guofangyi@ihep.ac.cn