Merge branch 'feature/update-to-clang-7' into 'master'

Update CI config to clang-7

See merge request !32

.gitlab-ci.yml

@@ -4,7 +4,7 @@ before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
 
 dune:git  clang  C++17:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-6-libcpp-17
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
   script: duneci-standard-test
 
 dune:git  gcc-8  C++17: