Create new unified Overlay transform

This MR creates a new unified Overlay transform targeting to merge MC and data overlay and be MT friendly. It is still in very early stages but SCT and truth overlays can be run with threads > 1. Please ignore commented-out code for now as things will probably still change a lot.

Current transforms should still work and the output should not change.

InnerDetector/InDetDigitization/SCT_Digitization/SCT_Digitization/ISCT_FrontEnd.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
 */
 
 /**
@@ -37,13 +37,6 @@ class ISCT_FrontEnd : virtual public ISiChargedDiodesProcessorTool {
   //** Destructor: */
   virtual ~ISCT_FrontEnd() {}
 
-  ///////////////////////////////////////////////////////////////////
-  // Pure virtual methods:
-  ///////////////////////////////////////////////////////////////////
-
-  //** process the collection of charged diodes */
-  virtual void process(SiChargedDiodeCollection& collection) const =0;
-  virtual void setRandomEngine(CLHEP::HepRandomEngine* rndmEngine) =0;
 };
 
 #endif // SCT_DIGITIZATION_ISCT_FRONTEND_H