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 )

Remove minimum required cmake version from DD4hepBuild, this interferes with other packages depending on DD4hep