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

 return empty vector instead of exception in LCDDImp::detectors(const string& type) if nothing found for type

SimpleDetector needs to link against DDRec

Split libraries into DDCore and DDComponents.
One has to explicitely specify the required components in find_package.
See doc/release.notes for more details

Simplify cmake: use add_dd4hep_plugin to create library and rootmap

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

Rename target to DDRec, this required changes in many other CMakeLists to be consistent

Re-arrange Gear dependent parts into single if scope
Add SYSTEM to GEAR IncludeDirs

Use variables to define libraries in one place and add optional ones (i.e., gear) to the list

Create plugin_sources variable to create a DDRecPlugin independent of the DDRec library parts

No Whitespace after and before parenthesis
Assume ROOT_INCLUDE_DIRS is added in main CMakeLists file
Comment Setting of CMAKE_CXX_FLAGS