Generators: workaround for SPGs
The SPG produces events with exactly one particle in the truth record and it doesn't set a beam particle, which throws off Rivet when it's doing the beam checks.
This MR will, in events with exactly one particle, set that single particle also as dummy beam particles. Together with the IgnoreBeamCheck = True
setting, this will be sufficient to pass Rivet's beam checks.
Needed for AGENE-2077
Edited by Christian Gutschow