Added one 'Geant4ScintillatorCalorimeterAction' into DDG4 plugins Geant4SDActions for  scintillator to take Geant4 BirksLaw effect as sensitive detector active layer.

  Overload function 'Geant4HitData::extractContribution' with 'bool ApplyBirksLaw' for extracting the MC contribution for a given hit from the step information with BirksLaw effect option. 

 Added a switch for BirksLaw. For backward compatable, by default, do not apply BirksLaw, and apply BirksLaw with call 'doApplyBirksLaw()'. 

 Added a BirksLaw function from Geant4 G4EmSaturation into DDG4 Geant4StepHandler. It may be called within function 'totalEnery()' for each G4Step.

including DD4hepG4Plugins, DD4hepG4LCIO, DD4hepG4Legacy
DDTest: replace deprecated link_libraries by target_link_libraries

Fixes for warnings from doxygen
proper list layout
verbatim around xml examples
escape unknown tag
remove copy/pasted doc for non-existent parameter

Create stringstream to avoid changing output format for cout

This also now allows to skip events

Require Geant 9.6 at CMAKE level
Replace "#pragma error" by "#error", because "#pragma error" is ignored as unknown

Macro needs string concatenation instead of quotation marks,
otherwise the plugin name comes out wrong

Fix hundreds of std=c++11 compiler warningsAllow to change compiler settings in cmake/DD4hep.cmake valid for all packages.Watch out for CMAKE_CXX_FLAGS in cmake/DD4hep.cmake.Move to use Boost spirit::phoenix version 3. ---- Prepare for ROOT 6