Add Pythia generator action

This adds a new command line option that allows to use Pythia to generate events on the fly.

FullSimLight/include/PythiaPrimaryGeneratorAction.hh

+#ifndef PythiaPrimaryGeneratorAction_h
+#define PythiaPrimaryGeneratorAction_h 1
+
+#if USE_PYTHIA
+
+#include <Pythia8/Pythia.h>
+#include <G4VUserPrimaryGeneratorAction.hh>
+
+class G4Event;
+
+bool use_pythia();
+void set_pythia_config(const char*);
+
+class PythiaPrimaryGeneratorAction final : public G4VUserPrimaryGeneratorAction {
+
+public:
+  PythiaPrimaryGeneratorAction();
+
+  void GeneratePrimaries(G4Event*) override;
+
+private:
+  Pythia8::Pythia pythia;
+};
+
+#endif
+#endif