Generators: weight-name patch when using HepMC3 release with HepMC2 GenEvent
It appears that the GenRunInfo is missing the weight names when using master--HepMC3 to read an EVNT produced with master--HepMC2. The weight name can be added manually, in case there is only one weight. When there are multiple weights, the error is passed on, though, since the interface doesn't know what variations the additional weights correspond to.
Observed in AGENE-2013.