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

added 11 commits

• 94100dab...c248f55d - 10 commits from branch cepc:master
• ee41ff60 - Merge branch CEPCSW:master into beambkg

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added 5 commits

• ee41ff60...de37af0e - 4 commits from branch cepc:master
• aaf5eab4 - Merge branch CEPCSW:master into beambkg

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added 19 commits

• aaf5eab4...c82aff1b - 18 commits from branch cepc:master
• 9a583cc5 - Merge branch CEPCSW:master into beambkg

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added 117 commits

• 9a583cc5...ea392988 - 116 commits from branch cepc:master
• 19be2efd - Merge branch CEPCSW:master into beambkg

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enabled an automatic merge when all merge checks for 19be2efd pass

changed title from Debug: sampling the beam particle number to Correct the beam particle number sampling in beam background simulation

changed the description

mentioned in commit ce475eb3

merged