Merge branch 'feature/remove-generic-geometry' into 'master'
remove generic geometry subdirectory On the last developer meeting, we agree on the following things: - The `genericgeometry` subdirectory should vanish in the next DUNE release. - All content of the namespace GenericGeometry is considered an implemenation detail. This merge implements these decisions. All functionality that is sitll required is migrated into the appropriate headers in `dune/geometry` and put into the namespace `Impl`. **Note**: There is *no transition phase* for the content of the `genericgeometry` subdirectory. See merge request !34