Propagate Pythia8 generator weight vector to EDM4hep

[selvaggi]

DelphesPythia8_EDM4HEP currently stores only the nominal event weight.

This PR implements storing the full pythia.info weight vector in EventHeader.weights and
and the weight labels in the metadata underedm4hep::labels::EventWeightsNames.

examples/data/ee_Z_bbbar_ecm91GeV_weights.cmd is an example card with the weight machinery enabled.

[kjvbrt]

Confirmed eventWeights()/eventWeightNames() on DelphesPythia8Reader also captures MadGraph LHE-level reweighting (<rwgt> blocks), not just Pythia8's own shower variations — Pythia8 merges both into info.weightValueVector()/weightNameVector() automatically when reading via Beams:LHEF. Verified against a real MadGraph-reweighted LHE file.

That means this PR subsumes #148.

[jmcarcell]

Do you want this in the upcoming release @selvaggi?

[selvaggi]

Do you want this in the upcoming release @selvaggi?

Yes, please.

[selvaggi]

@jmcarcell @kjvbrt anything else before this can be merged?

[tmadlener]

This looks good to me. I suppose the weight names follow from the pythia cmd file somehow? In light of @kjvbrt comment that this also includes the LHEF weights, I think it would be a good idea to also add a test related to that (potentially in a follow up PR).

I have rebased onto the current main branch and resolved a minor conflict along the way.

[selvaggi]

@tmadlener yes the names come from pythia.info.weightNameVector() so they follow directly from the cmd file. The entry 0 is always the Pythia nominal weight. For each group defined in UncertaintyBands:List / VariationFrag:list Pythia books one weight named by the group label from the card plus one per individual keyword with an autogenerated name.