Hepmc3 nightly fixes 03122020 part 1

In this MR:

• Use auto to match GenParticlePtr/GenParticle*
• Use C++11 loops for HEpMC3 compatibility
• use ifdef to deal with sysntax update for particles_size() vs particles().size()
• use free function barcode for HepMC3 compatibility

Calorimeter/CaloLocalHadCalib/src/GetLCDeadMaterialTree.cxx

@@ -222,8 +222,12 @@ StatusCode GetLCDeadMaterialTree::execute()
   ******************************************** */
   const McEventCollection* truthEvent=nullptr;
   ATH_CHECK( evtStore()->retrieve(truthEvent, "TruthEvent") );
+#ifdef HEPMC3
+  HepMC::ConstGenParticlePtr gen  = truthEvent->at(0)->particles().front();
+#else
   HepMC::GenEvent::particle_const_iterator pit  = truthEvent->at(0)->particles_begin();
   const HepMC::GenParticle * gen  = *pit;
+#endif
 
   double mc_eta = gen->momentum().pseudoRapidity();
   double mc_phi = gen->momentum().phi();