Add diffraction-tuned pythia8 inel generator

MC/config/common/ini/pythia8_inel_difftuned.ini

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+[GeneratorPythia8]
+config=${O2DPG_MC_CONFIG_ROOT}/MC/config/common/pythia8/generator/pythia8_inel_difftuned.cfg

MC/config/common/ini/tests/pythia8_inel_difftuned.C

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+int pythia8() {
+  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();
+  if (nEvents == 0) {
+    std::cerr << "No event of interest\n";
+    return 1;
+  }
+  return 0;
+}

MC/config/common/pythia8/generator/pythia8_inel_difftuned.cfg

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+### beams
+Beams:idA 2212			# proton
+Beams:idB 2212 			# proton
+Beams:eCM 13600. 		# GeV
+
+### processes
+SoftQCD:inelastic on		# all inelastic processes
+
+### decays
+ParticleDecays:limitTau0 on	
+ParticleDecays:tau0Max 10.	
+
+### adjust diffraction
+### note that PYTHIA calculates the ND sigma via a
+### 'remaining from total' formula: no need to put
+### that in by hand
+
+SigmaTot:mode 0
+SigmaDiffractive:mode 0
+SigmaTotal:sigmaTot 78.6
+SigmaTotal:sigmaEl 0.0
+SigmaTotal:sigmaXB 10.625
+SigmaTotal:sigmaAX 10.625
+SigmaTotal:sigmaXX 9.78
+SigmaTotal:sigmaAXB 0.0