Matching/Merging and weight propagation to HepMC files

[TehMeh]

Hi,

in some cases, MadGraph uses matching/merging (MLM, FxFx, etc.) procedures. The result is that some events are removed due to double counting and there are more events in lhe files than in HepMC or ROOT files. If that is the case, then it is impossible to read weight labels/ids from lhe files and the only option is to use weights='delphes' option when adding samples.

However, currently, MadGraph makes it difficult to read weights from HepMC files.
MadGraph/Pythia correctly saves the numerical value (with some extra factors), but the weight id/label is a stylized version of set param_card <block> <id> <value> # orig <orig_value>.
To overcome this, it is needed to slightly edit <MG_directory>/HEPTools/MG5aMC_PY8_interface/MG5_PY8_interace.cc so that the actual weight ids are read from lhe files and propagated to HepMC files.
Changes to the MadMiner interface to HepMC files are also needed to accommodate the correctly written weight ids.