add generator for NeNe based on EPOS4

MC/config/examples/ini/GeneratorEPOS4NeNe536TeV.ini

@@ -0,0 +1,12 @@
+#NEV_TEST> 10
+[GeneratorExternal]
+fileName=${O2DPG_MC_CONFIG_ROOT}/MC/config/examples/epos4/generator_EPOS4.C
+funcName=generateEPOS4("${O2DPG_MC_CONFIG_ROOT}/MC/config/examples/epos4/generator/NeNe_536TeV_EPOS4.optns", 2147483647)
+
+[GeneratorFileOrCmd]
+cmd=${O2DPG_MC_CONFIG_ROOT}/MC/config/examples/epos4/epos.sh
+bMaxSwitch=none
+
+# Set to version 2 if EPOS4.0.0 is used
+[HepMC]
+version=3

MC/config/examples/ini/tests/GeneratorEPOS4NeNe536TeV.C

@@ -0,0 +1,78 @@
+int External()
+{
+  std::string path{"o2sim_Kine.root"};
+
+  // Check that file exists, can be opened and has the correct tree
+  TFile file(path.c_str(), "READ");
+  if (file.IsZombie())
+  {
+    std::cerr << "Cannot open ROOT file " << path << "\n";
+    return 1;
+  }
+
+  auto tree = (TTree*)file.Get("o2sim");
+  if (!tree)
+  {
+    std::cerr << "Cannot find tree o2sim in file " << path << "\n";
+    return 1;
+  }
+
+  std::vector<o2::MCTrack>* tracks{};
+  tree->SetBranchAddress("MCTrack", &tracks);
+
+  // Check if all events are filled
+  auto nEvents = tree->GetEntries();
+  for (Long64_t i = 0; i < nEvents; ++i)
+  {
+    tree->GetEntry(i);
+    if (tracks->empty())
+    {
+      std::cerr << "Empty entry found at event " << i << "\n";
+      return 1;
+    }
+  }
+
+  // Check if there is 1 event, as customarily set in the ini file
+  // Heavy-ion collisions with hydro and hadronic cascade are very slow to simulate
+  if (nEvents != 10)
+  {
+    std::cerr << "Expected 10 events, got " << nEvents << "\n";
+    return 1;
+  }
+
+  // ---- Neon-Neon parameters ----
+  constexpr int kNeonPDG = 1000100200;     // Ne-20 ion
+  constexpr double kEnucleon = 5360.;      // GeV per nucleon
+  constexpr int kA = 20;                   // Neon mass number
+  constexpr double kNeonEnergy = kA * kEnucleon / 2.0; // beam energy in GeV
+
+  // Check if each event has two neon ions at expected energy
+  for (int i = 0; i < nEvents; i++)
+  {
+    tree->GetEntry(i);
+    int count = 0;
+
+    for (int idxMCTrack = 0; idxMCTrack < tracks->size(); ++idxMCTrack)
+    {
+      auto track = tracks->at(idxMCTrack);
+      double energy = track.GetEnergy();
+
+      // 50 MeV tolerance (floating point safety)
+      if (std::abs(energy - kNeonEnergy) < 5e-2 &&
+          track.GetPdgCode() == kNeonPDG)
+      {
+        count++;
+      }
+    }
+
+    if (count < 2)
+    {
+      std::cerr << "Event " << i
+                << " has less than 2 neon ions at "
+                << kNeonEnergy << " GeV\n";
+      return 1;
+    }
+  }
+
+  return 0;
+}