Todos: - [x] Update Dumux-version from 3.4 to 3.5 - [x] Introduce a macro for the Dune version and update Dune-versions from 2.7 to 2.8 - [x] Correct typos and improve phrasing - [x] Check changelog if there is anything necessary to include into the handbook - nothing necessary - [x] Correct control volume for cell-centered primary variables in fig. 6.7 Corresponding MR: !3032