Explicitly request visitor from the ::Impl namespace
Rather than just writing 'Impl'. This makes the CI system pass for
https://gitlab.mn.tu-dresden.de/paraphase/dune-fracture-phasefields/merge_requests/9
Rather than just writing 'Impl'. This makes the CI system pass for
https://gitlab.mn.tu-dresden.de/paraphase/dune-fracture-phasefields/merge_requests/9