More consistently apply external ion pdg table

It appears that sometimes the external ion table
was not loaded (only when users got hold of the singleton instance).
This should now be fixed.

DataFormats/simulation/include/SimulationDataFormat/O2DatabasePDG.h

@@ -38,17 +38,14 @@ class O2DatabasePDG
     auto db = TDatabasePDG::Instance();
     if (!initialized) {
       addALICEParticles(db);
-      if (const char* o2Root = std::getenv("O2_ROOT")) {
-        auto inputExtraPDGs = std::string(o2Root) + "/share/Detectors/gconfig/data/extra_ions_pdg_table.dat";
-        db->ReadPDGTable(inputExtraPDGs.c_str());
-      }
       initialized = true;
     }
     return db;
   }
 
   // adds ALICE particles to a given TDatabasePDG instance
   static void addALICEParticles(TDatabasePDG* db = TDatabasePDG::Instance());
+  static void addParticlesFromExternalFile(TDatabasePDG* db);
 
   // get particle's (if any) mass
   static Double_t MassImpl(TParticlePDG* particle, bool& success)
@@ -655,6 +652,26 @@ inline void O2DatabasePDG::addALICEParticles(TDatabasePDG* db)
   if (!db->GetParticle(-ionCode)) {
     db->AddParticle("AntiSexaquark", "AntiSexaquark", 2.0, kTRUE, 0.0, 0, "Special", -ionCode);
   }
+
+  // lastly, add particle from the the extra text file
+  addParticlesFromExternalFile(db);
+}
+
+inline void O2DatabasePDG::addParticlesFromExternalFile(TDatabasePDG* db)
+{
+  static bool initialized = false;
+  if (!initialized) {
+    // allow user to specify custom file
+    if (const char* custom = std::getenv("O2_SIM_CUSTOM_PDG")) {
+      // TODO: make sure this is a file
+      db->ReadPDGTable(custom);
+    } else if (const char* o2Root = std::getenv("O2_ROOT")) {
+      // take the maintained file from O2
+      auto inputExtraPDGs = std::string(o2Root) + "/share/Detectors/gconfig/data/extra_ions_pdg_table.dat";
+      db->ReadPDGTable(inputExtraPDGs.c_str());
+    }
+    initialized = true;
+  }
 }
 
 } // namespace o2