First implementation of dipole field, HepMC reader for DDG4 and fix to import constants into DDG4 python

 - implemented for VXDSupportMaterial using 
   MaterialManager::createAveragedMaterial()

 - added creation of BeamPipe Gear paramaters

   for 'static const int' defined in gear header file 

 - added treatment of gear::FTDParameters (for ILD )

   following the change in Detector.cpp

 - implemented for the VXD

   Gear objects from DetElements with 
   data structures holding relevant information
   

 - fixed relative pathes to pictures and manuals in
   Doxygen main page

   a DDG4 based simulation with xml configuration files
   (originally in DDSim)
 - renamed print_materials.cc to print_materials.cpp

   - don't write particles created in calo
     unless scattered back into tracker)
   - don't write particles created in tracker
     if below MinimalKineticEnergy ( Geant4ParticleHandler)