Persist and add to metadata contained in hepmc run attributes

[simonge]

Run attributes, at least from the signal file, should be kept in the merged file such as the beam settings added by the afterburner. Additional metadata could be added such as the length of the time slice and the total slice luminosity either as attributes on the run or event level.

Checks could be added to make sure that the signal and background files are from the same beam settings so unphysical merging is stopped. This will require the background generators to add this information to the hepmc file.