Move mass sampling from particle gun to FlatPtRapidity
Closes LHCBGAUSS-1350
Port merging of branch 'johndan-MoveMassSampling' into 'Sim09-upgrade'
Correspond to merge request !268 (merged) into 'master' and !380 (merged) into 'Sim09'
(cherry picked from commit 9c94d053)
8f3a12c6 Move mass sampling from particle gun to FlatPtRapidity