ISF_FatrasToolsG4: Flush G4cout/cerr streams in G4HadIntProcessor by-hand

Force flush Geant4 stream buffers to work around the problem discussed in ATLASSIM-5137. It seems to work locally but we should keep an eye on the ART tests to confirm.

cc: @jchapman @ssnyder

Simulation/ISF/ISF_Fatras/ISF_FatrasToolsG4/src/G4HadIntProcessor.cxx

@@ -54,6 +54,7 @@
 #include "G4IsotopeVector.hh"
 #include "G4Neutron.hh"
 #include "G4ProductionCutsTable.hh"
+#include "G4ios.hh"
 
 // CLHEP
 #include "CLHEP/Units/SystemOfUnits.h"
@@ -370,6 +371,10 @@ bool iFatras::G4HadIntProcessor::initG4RunManager() const {
 
   //G4cout << *(G4Material::GetMaterialTable()) << std::endl;
 
+  // Flush the G4 cout/cerr: ATLASSIM-5137
+  G4cout << std::flush;
+  G4cerr << std::flush;
+
   return true;
 }