[PWGEM] Add configs for Dalitz-decay MC

MC/config/PWGEM/ini/pythia8_pp_13600_ForcedDalitz.ini

@@ -0,0 +1,5 @@
+[GeneratorPythia8]
+config=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGEM/pythia8/generator/configPythia_ForcedDalitz.cfg
+
+[DecayerPythia8]
+config[0]=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGEM/pythia8/decayer/force_dummy.cfg

MC/config/PWGEM/ini/tests/pythia8_pp_13600_ForcedDalitz.C

@@ -0,0 +1,39 @@
+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;
+      }
+    }
+
+
+    return 0;
+  }
+
+  int pythia8()
+  {
+      return External();
+  }
+

MC/config/PWGEM/pythia8/generator/configPythia_ForcedDalitz.cfg

@@ -0,0 +1,21 @@
+ProcessLevel:all on       
+
+### beams
+Beams:idA 2212			# proton
+Beams:idB 2212 			# proton
+Beams:eCM 13600. 		# GeV
+
+### processes
+SoftQCD:inelastic on		# all inelastic processes
+
+ParticleDecays:limitTau0 on
+ParticleDecays:tau0Max  10     
+
+
+#decay eta -> e+ e- gamma BR = 100%
+221:onMode = off
+221:onIfMatch = 22 11 -11
+#221:addChannel = 1 1 0 22 11 -11
+
+#decay pi0 -> e+ e- gamma BR = 10%
+111:addChannel = 1 0.10 0 22 11 -11"