build all examples in one go
    - made necessary changes in package cmake files
      for this to work
    - updated documentation in Readme

Add root libs to (DY)LD_LIBRARY_PATH, make cmake find genmap. No longer need to have rootsys/bin in the path.

   - use version suggested by A.Sailer for all but macos (APPLE)
   - added extra function dd4hep_generate_rootmap_apple()
     this version sources the thisXXX.sh scripts
     before running genmap - but does not seem to work for Linux
     -> needs to be resolved ...