User action that sets G4 looper threshold values

This user action can configure the looper threshold values in Geant4: http://www-geant4.kek.jp/lxr/source/processes/transportation/src/G4Transportation.cc#L878

The preInclude will set them both to 0 for debugging purposes. By default, Geant4 would only print Exceptions for particles above 100 MeV.

The user action also calls a Geant4 printout method, e.g.:

G4Transportation:  Current values for thresholds related to  the killing of looping tracks:
Warning Energy   = 0 MeV   ( below this tracks are killed without warning )
Important Energy = 0  ( above this tracks are given multiple chances )
Extra Trials     = 10 'important' tracks, i.e. those above 'important' energy

Simulation/G4Utilities/G4UserActions/G4UserActions/LooperThresholdSet.h

+/*
+   Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+ */
+
+#ifndef G4UserActions_LooperThresholdSet_H
+#define G4UserActions_LooperThresholdSet_H
+
+#include "G4UserRunAction.hh"
+#include "AthenaBaseComps/AthMessaging.h"
+
+#include "G4Run.hh"
+#include "G4ParticleDefinition.hh"
+#include "G4Transportation.hh"
+#include "G4CoupledTransportation.hh"
+#include "G4SystemOfUnits.hh"
+
+namespace G4UA
+{
+
+class LooperThresholdSet : public AthMessaging, public G4UserRunAction
+{
+public:
+
+struct Config
+{
+        double WarningEnergy = 100.0 * CLHEP::MeV;
+        double ImportantEnergy = 250.0 * CLHEP::MeV;
+        int NumberOfTrials = 10;
+};
+
+LooperThresholdSet( const Config& config );
+virtual void BeginOfRunAction( const G4Run* ) override;
+
+private:
+
+Config m_config;
+
+void ChangeLooperParameters( const G4ParticleDefinition* particleDef );
+std::pair<G4Transportation*, G4CoupledTransportation*> findTransportation( const G4ParticleDefinition* particleDef );
+};   // class LooperThresholdSet
+
+} // namespace G4UA
+
+#endif