Config file improvements
Use configure_package_config_file instead of configure_file to generate HepMC3-config - it handles absolute paths properly.
Use HEPMC3_ROOTIO_INSTALL_LIBDIR when generating HepMC3Config.cmake
Use configure_package_config_file instead of configure_file to generate HepMC3-config - it handles absolute paths properly.
Use HEPMC3_ROOTIO_INSTALL_LIBDIR when generating HepMC3Config.cmake