Converter: propagate original GenRunInfo for events after the first

[veprbl]

Fixes: #19

[kkauder]

Interesting. In the past, I have used

    GenEvent hepmc3evt( HepMC3::Units::GEV, HepMC3::Units::CM );
...
    GenCrossSectionPtr xsec = std::make_shared<GenCrossSection>();
    xsec->set_cross_section( crossSection, crossSectionError);
    hepmc3evt.set_cross_section(xsec);