Add empty external generator and adjust ini files (#1603)

Co-authored-by: jesgum <je8022ka-s@student.lu.se>

Author: jesgum

MC/config/ALICE3/ini/pythia8_ArAr.ini

@@ -1,5 +1,9 @@
 [Diamond]
 width[2]=6.0
 
+[GeneratorExternal]
+fileName=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C
+funcName=generator_pythia8_ALICE3()
+
 [GeneratorPythia8]
 config=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_ArAr.cfg

MC/config/ALICE3/ini/pythia8_KrKr.ini

@@ -1,5 +1,9 @@
 [Diamond]
 width[2]=6.0
 
+[GeneratorExternal]
+fileName=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C
+funcName=generator_pythia8_ALICE3()
+
 [GeneratorPythia8]
 config=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_KrKr.cfg

MC/config/ALICE3/ini/pythia8_OO.ini

@@ -1,5 +1,9 @@
 [Diamond]
 width[2]=6.0
 
+[GeneratorExternal]
+fileName=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C
+funcName=generator_pythia8_ALICE3()
+
 [GeneratorPythia8]
 config=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_OO.cfg

MC/config/ALICE3/ini/pythia8_PbPb.ini

@@ -1,5 +1,9 @@
 [Diamond]
 width[2]=6.0
 
+[GeneratorExternal]
+fileName=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C
+funcName=generator_pythia8_ALICE3()
+
 [GeneratorPythia8]
 config=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_PbPb.cfg

MC/config/ALICE3/ini/pythia8_XeXe.ini

@@ -1,5 +1,9 @@
 [Diamond]
 width[2]=6.0
 
+[GeneratorExternal]
+fileName=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C
+funcName=generator_pythia8_ALICE3()
+
 [GeneratorPythia8]
 config=${O2DPG_ROOT}/MC/config/ALICE3/pythia8/generator/pythia8_XeXe.cfg

MC/config/ALICE3/pythia8/generator_pythia8_ALICE3.C

@@ -0,0 +1,31 @@
+
+#if !defined(__CLING__) || defined(__ROOTCLING__)
+#include "Pythia8/Pythia.h"
+#include "FairGenerator.h"
+#include "FairPrimaryGenerator.h"
+#include "Generators/GeneratorPythia8.h"
+#include "TRandom3.h"
+#include "TParticlePDG.h"
+#include "TDatabasePDG.h"
+#include "TMath.h"
+#include <cmath>
+using namespace Pythia8;
+#endif
+
+// Default pythia8 minimum bias generator
+// Please do not change
+
+class GeneratorPythia8ALICE3 : public o2::eventgen::GeneratorPythia8
+{
+public:
+  /// Constructor
+  GeneratorPythia8ALICE3() {}
+
+  ///  Destructor
+  ~GeneratorPythia8ALICE3() = default;
+};
+
+ FairGenerator *generator_pythia8_ALICE3() 
+ {
+   return new GeneratorPythia8ALICE3();
+ }