update MG version to 2.7.2p3.atlas5 and fix TestHepMC bug
update MG version to 2.7.2p3.atlas5 fix TestHepMC bug connected with tau decay length requirement, as discussed in https://its.cern.ch/jira/browse/AGENE-1860
update MG version to 2.7.2p3.atlas5 fix TestHepMC bug connected with tau decay length requirement, as discussed in https://its.cern.ch/jira/browse/AGENE-1860