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.

 - enabling this code breaks some tests in the examples
   so we disable it for now

   => to be fixed next year

 - still need resolution for failing tests
  - or revert to old treatment of elements ...
 - release notes created from
    git log --pretty=format:"%aN %ad %n  - %s" --date=short
   => should have squashed commits with more
   meaningful commit notes in the future ...

add map cellID of surface <--> vector of cellID of neighbouring surfaces for ZPlanar and ZPetalDisk data struct

 - also set DD4HEP_LIBRARYPATH in dd4hep_add_library_path()
 - export ROOTSYS if it has not been set
    ( assumes path_to_ROOT/cmake )