[ALICE 3] Add generator and ini files for PbPb 5.52 TeV with more abundant Xi (#1973)

* Add configuration for PbPb 5.52TeV with more abundant Xi and Omegas

* Update test

Author: jesgum

MC/config/ALICE3/ini/tests/xicc_background_PbPb.C

@@ -0,0 +1,38 @@
+int External() 
+{
+  std::string path{"o2sim_Kine.root"};
+
+  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);
+  auto nEvents = tree->GetEntries();
+  int nInjected = 0;
+  for (int i = 0; i < nEvents; i++) {
+    tree->GetEntry(i);
+    for (auto& track : *tracks) {
+      auto pdgCode = std::fabs(track.GetPdgCode());
+      if (pdgCode == 3312) {
+        nInjected++;
+      }
+    }
+  }
+
+  // Check if we are above typical Angantyr numbers
+  if (nInjected < 5 * nEvents) {
+    std::cerr << "Too few particles injected\n";
+    return 1;
+  }
+
+  return 0;
+}

MC/config/ALICE3/ini/xicc_background_PbPb.ini

@@ -0,0 +1,9 @@
+[Diamond]
+width[2]=6.0
+
+[GeneratorExternal]
+fileName=${O2DPG_MC_CONFIG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_strangeness_gun_PbPb.C
+funcName=generatePYTHIA()
+
+[GeneratorPythia8]
+config=${O2DPG_MC_CONFIG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_PbPb.cfg