Fix collisions in merge with changes from Frank.

Fix example DDG4/examples/readHEPMC.py and allow access to STL containers from DD4hep python interface

Initialise variable for surface list
remove useless calls and comment argument name to avoid unused argument warning

 - changed signature of Geant4EventReader::readParticles(int,Vertex*, Particles&)
   to Geant4EventReader::readParticles(int,Vertices&, Particles& )
 - implement in LCIOEventReader, Geant4EventReaderHepEvt and Geant4EventReaderHepMC
 - for now still one vertex only is created using the first final state particle
   for HepEvt and LCIO

Add possibility to remove the production cut from region so that it uses the Global Range Cut instead of the default set for Regions

Geant4 will print a warning about regions without thresholds, but that is what we need. This allows one to keep regions for e.g., creating material scan with out manually having to set range cuts
As far as I can tell the threshold is only used when store_secondaries is true (which is ignored), so we are now throwing an exception if someone specifies store_secondaries without setting a threshold.
However, it seems that store_secondaries is actually never used anywhere else?

TSystem::Load return > 0 if the library is already loaded. This is not an error. For some reason it was already loaded when I run ddsim in a singularity container. I have no idea why.

Having all indentation made browsing the generated doxygen source code
unbearable.

Also updated the install path for the cmake files so it doesn't pollute
the install prefix.

	modified:   cmake/DD4hepMacros.cmake
	modified:   doc/doxygen/DD4hep.css
	modified:   doc/doxygen/DD4hepFooter.html
	modified:   doc/doxygen/DD4hepHeader.html

pick the VolumeManager from lcdd, and create it if necessary instead of creating a static one in SurfaceHelper
This is necessary in case someone needs a volumemanger before surfaceHelper is called.