- fix ignoring input lines with 'nan'
 - did not work on SL6 w/ gcc

 Added one more if statement: If the track went into new Volume, then extracted the hit in previous Volume, and start a new hit in this current Volume. And in this current process, also allow the same following treatments for the new hit.

Update doxygen information for some undocumented classes. Add licence header to files where not present.

Keep sensitive detector type constant. Add property to Geant4Kernel if special/user defined G4VSensitiveDetector instances should be created

Implement issues https://github.com/AIDASoft/DD4hep/issues/373, https://github.com/AIDASoft/DD4hep/issues/371

Previously was overloading operator(), now is constructField()

This addresses issue #376.  Previously, the initial m_refCount=1 was not
being accounted for

Also, this commit removes unnecessary readoption of the Geant4Actions

Geant4Output2LCIO: Use event and run number offset also in the case of presence of input event parameters extension

Geant4Output2LCIO: get LCIO input event parameters from dedicated event extension (if exists) and write them to LCIO output event

Geant4InputAction and Geant4EventReader: register input action into event reader enabling the event reader to access the Geant4 context

 - in Geant4InputHandling.cpp

DDG4::InputHandling pass also particles with 0 lifetime to geant, only ignore quarks, gluons and 'strings'