Fix missing 2->2 vertices in Herwig7/HepMC3

For some unknown reason, sometimes Herwig7 has missing vertices for n->m processes. In the past, it was seen for 2->1 and 1->2 and the FixHepMC algorithm had fixes for those situations. But recently it appears to be the case for the 2->2 processes (leptoquark production). The issue was reported here AGENE-2197, but unfortunately, it was not reproducible outside of Athena, therefore so far the way to go is to amend the FixHepMC and allow it to apply the fixes for the case of 2->2.

Tag @ewelina @dbuchin @dhirsch

Use urgent because the affected samples are needed fro an analysis in progress.