LHCBGAUSS-1297.FixedMass.Sim09
Port merge of branch 'LHCBGAUSS-1297.FixedMass.Sim09'
See merge request !269 (merged) into 'Sim09', corresponds to !252 (merged) into master that had to be done by hand due to the many changes in this code for FSR and new Pythia8 version
(cherry picked from commit 322bda8e)
73e856d5 Initial fix for mass width bug, now based on Sim09 branch.