Add LHEF event weights to edm4hep output

[vvolkl]

in DelphesPythia8Reader, DelphesPythia8EvtgenReader

[tmadlener]

#5 in principle makes the LHEFEvent and LHEFWeight branches available to the converter.

What is the information that should be stored, resp. is there a preferred way to store it? E.g. should we (ab)use metadate or use another ParticleIDcollection with just one entry per event?