Geant4AssemblyVolume: use the same copy ID for the Geant4 Volume as was set in DD4hep geometry construction

Add implementation for signal handling. Use generic signal handler in DDG4 for controlled shutdown on CTRL-C. Add test DDG4_SIGINT_handler in examples/DDG4

Move generator status setting to common place

GenerationActionInit: declare properties to access number of processed runs and events

when the value is used as 'half-length'
replacing GetNegativeEndZ by GetPositiveEndZ fixes the problem
because GetPositiveEndZ is the user value that corresponds to half-length

Explicity set namespace to TrackClassification in
TestStackingAction::classifyNewTrack

Remove support for very old versions of ROOT < 6.26.0. Remove default use of std namespace from some implementation files.