[PWGLF] Added code for flat phase-space MC generator for PWA study

MC/config/PWGLF/ini/generatorLF_Resonances_flat_mass.ini

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+[GeneratorExternal]
+fileName=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGLF/pythia8/generator_pythia8_LF_flat_mass.C
+funcName=generateLFRapidity("${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGLF/pythia8/generator/flatMCgun.json", true, 0, false, true, "${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGLF/pythia8/generator/pythia8_inel_136tev.cfg", "")
+
+[GeneratorPythia8] # Background event config
+config=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGLF/pythia8/generator/pythia8_inel_136tev.cfg
+
+[DecayerPythia8] # Transport decayer config
+config[0]=${O2DPG_MC_CONFIG_ROOT}/MC/config/common/pythia8/decayer/base.cfg
+config[1]=${O2DPG_MC_CONFIG_ROOT}/MC/config/PWGLF/pythia8/generator/resonances.cfg

MC/config/PWGLF/ini/tests/generatorLF_Resonances_flat_mass.C

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+int External()
+{
+    std::string path{"o2sim_Kine.root"};
+
+    //Kept only target single resonance (e.g. PDG 10331 -> K0S K0S)
+    std::vector<int> injectedPDGs = {10331};
+    std::vector<std::vector<int>> decayDaughters = {{310, 310}};
+
+    int numberOfInjectedSignalsPerEvent = 5;
+    int numberOfEventsProcessed = 0;
+    int numberOfEventsProcessedWithoutInjection = 0;
+
+    auto nInjection = injectedPDGs.size();
+
+    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);
+
+    std::vector<int> nSignal(nInjection, 0);
+    std::vector<std::vector<int>> nDecays;
+    std::vector<int> nNotDecayed(nInjection, 0);
+
+    for (size_t i = 0; i < nInjection; i++)
+    {
+        std::vector<int> nDecay(decayDaughters[i].size(), 0);
+        nDecays.push_back(nDecay);
+    }
+
+    auto nEvents = tree->GetEntries();
+    bool hasInjection = false;
+
+    for (int i = 0; i < nEvents; i++)
+    {
+        hasInjection = false;
+        numberOfEventsProcessed++;
+        tree->GetEntry(i);
+
+        for (size_t idxMCTrack = 0; idxMCTrack < tracks->size(); ++idxMCTrack)
+        {
+            auto track = tracks->at(idxMCTrack);
+            auto pdg = track.GetPdgCode();
+            auto it = std::find(injectedPDGs.begin(), injectedPDGs.end(), pdg);
+
+            if (it != injectedPDGs.end()) // Found injected mother particle
+            {
+                int index = std::distance(injectedPDGs.begin(), it);
+                nSignal[index]++;
+
+                if (track.getFirstDaughterTrackId() < 0)
+                {
+                    nNotDecayed[index]++;
+                    continue;
+                }
+
+                for (int j{track.getFirstDaughterTrackId()}; j <= track.getLastDaughterTrackId(); ++j)
+                {
+                    auto pdgDau = tracks->at(j).GetPdgCode();
+                    bool foundDau = false;
+
+                    for (size_t idxDaughter = 0; idxDaughter < decayDaughters[index].size(); ++idxDaughter)
+                    {
+                        if (pdgDau == decayDaughters[index][idxDaughter])
+                        {
+                            nDecays[index][idxDaughter]++;
+                            foundDau = true;
+                            hasInjection = true;
+                            break;
+                        }
+                    }
+                    if (!foundDau)
+                    {
+                        std::cerr << "Decay daughter not found: " << pdg << " -> " << pdgDau << "\n";
+                    }
+                }
+            }
+        }
+        if (!hasInjection)
+        {
+            numberOfEventsProcessedWithoutInjection++;
+        }
+    }
+
+    std::cout << "--------------------------------\n";
+    std::cout << "# Events: " << nEvents << "\n";
+    for (size_t i = 0; i < nInjection; i++)
+    {
+        std::cout << "# Mother PDG: " << injectedPDGs[i] << "\n";
+        std::cout << "  Generated: " << nSignal[i] << ", " << nNotDecayed[i] << " did not decay\n";
+
+        if (nSignal[i] == 0)
+        {
+            std::cerr << "No generated signal found for PDG: " << injectedPDGs[i] << "\n";
+        }
+        for (size_t j = 0; j < decayDaughters[i].size(); j++)
+        {
+            std::cout << "  Daughter PDG " << decayDaughters[i][j] << ": " << nDecays[i][j] << "\n";
+        }
+    }
+    std::cout << "--------------------------------\n";
+    std::cout << "Number of events processed: " << numberOfEventsProcessed << "\n";
+    std::cout << "Number of events without injection: " << numberOfEventsProcessedWithoutInjection << "\n";
+
+    return 0;
+}
+
+void generatorLF_Resonances_flat_mass() { External(); }

MC/config/PWGLF/pythia8/generator/flatMCgun.json

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+{
+    "f0(1710)" : {
+        "pdg": 10331,
+        "n": 100,
+        "ptMin": 0.0,
+        "ptMax": 30,
+        "rapidityMin": -1.0,
+        "rapidityMax": 1.0,
+        "massMin": 1.0,
+        "massMax": 3.0,
+        "genDecayed": true
+    }
+}