Migrate EvgenProdTools to HepMC3

In this MR:

• Migrate EvgenProdTools to HepMC3

Generators/AtlasHepMC/AtlasHepMC/Relatives.h

+/* Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+   Author: Andrii Verbytskyi andrii.verbytskyi@mpp.mpg.de
+*/
+#ifndef ATLASHEPMC_RELATIVES_H
+#define ATLASHEPMC_RELATIVES_H
+#ifdef HEPMC3
+#include "HepMC3/Relatives.h"
+namespace HepMC {
+typedef HepMC3::Relatives Relatives;
+using HepMC3::children_particles;
+using HepMC3::children_vertices;
+using HepMC3::grandchildren_particles;
+using HepMC3::grandchildren_vertices;
+using HepMC3::parent_particles;
+using HepMC3::parent_vertices;
+using HepMC3::grandparent_particles;
+using HepMC3::grandparent_vertices;
+using HepMC3::descendant_particles;
+using HepMC3::descendant_vertices;
+using HepMC3::ancestor_particles;
+using HepMC3::ancestor_vertices;
+}
+#endif
+#endif
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