Apply clang tidy

After !10 is merged, we should apply clang-tidy as I did in dune-xt!92 (merged) and dune-xt!94 (merged). The infrastructure is already in place (assuming we are using the recent dune-xt master branch), we only have to update .vcsetup and can then just run ninja -j8 fix_tidy_parallel in the build directory (or maybe just ninja fix_tidy if the parallel fixes cause lots of errors). Maybe we should apply the -modernize-use-override, modernize-concat-nested-namespaces and readability-else-after-return checks first in separate tidy runs (by replacing the Checks: list in .vcsetup/.clang-tidy by the respective check) and commit the results after each run as these checks introduce a lot of changes.