combine all VolIDs from all parent volumes
     in the path to the current sensitive volume
    - used in Geant4CalorimeterSD and 
      Geant4TrackerSD

  
  - added new method:
    long long Geant4SensitiveDetector::getVolumeID(G4Step* step)
  
  - use it in Geant4Tracker(Calorimeter)SD
  
   - changed ILDExEventAction to extract the correct 
     readout string from lookup via cellID
 

 - changed to use standard way of finding geant4 with CMake:
   -D Geant4_DIR=_path_to_Geant4Config.cmake 
   (this script provies also the dependencies used to build geant4)

  - added Property $Id$ to all *.h *.cpp and *.xml files
    to allow versioning with SVN