GeoModelExamples: Avoid use of non-bound DataHandle.

Deprecating use of non-bound DataHandle, especially non-const.
Also adding const for materials.

DetectorDescription/GeoModel/GeoModelExamples/src/ToyDetectorFactory.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "ToyDetectorFactory.h"
@@ -21,7 +21,6 @@
 #include "CLHEP/GenericFunctions/Sin.hh"
 #include "CLHEP/GenericFunctions/Cos.hh"
 #include "StoreGate/StoreGateSvc.h"
-#include "StoreGate/DataHandle.h"
 
 #include "GeoModelInterfaces/StoredMaterialManager.h"
 using namespace Genfun;
@@ -50,7 +49,7 @@ void ToyDetectorFactory::create(GeoPhysVol *world)
 {
   m_detectorManager=new ToyDetectorManager();
 
-  DataHandle<StoredMaterialManager> materialManager;
+  const StoredMaterialManager* materialManager = nullptr;
   if (StatusCode::SUCCESS != m_detectorStore->retrieve(materialManager, std::string("MATERIALS"))) {
     return; 
   } 

DetectorDescription/GeoModel/GeoModelExamples/src/ToyDetectorTool.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "ToyDetectorTool.h"
@@ -42,7 +42,7 @@ ToyDetectorTool::create()
   // 
   // Locate the top level experiment node 
   // 
-  DataHandle<GeoModelExperiment> theExpt; 
+  GeoModelExperiment* theExpt = nullptr;
   if (StatusCode::SUCCESS != detStore()->retrieve( theExpt, "ATLAS" )) { 
     log << MSG::ERROR 
 	<< "Could not find GeoModelExperiment ATLAS"