Add support for reading McEventCollection_p6 to 21.0
Part of the work for ATLINFR-4725.
NB Any HepMC3 attributes including filterWeight will be lost when reading in McEventCollection_p6.
Edited by John Derek Chapman
Part of the work for ATLINFR-4725.
NB Any HepMC3 attributes including filterWeight will be lost when reading in McEventCollection_p6.