Feature/interpolation

Interpolation between grids, initial version. Added traversal over irregular elements and gitlabci tests

.gitlab-ci.yml

 ---
 before_script:
-  - . /duneci/bin/duneci-init-job
-  - duneci-install-module https://gitlab.dune-project.org/core/dune-common.git
-  - duneci-install-module https://gitlab.dune-project.org/core/dune-geometry.git
-  - duneci-install-module https://gitlab.dune-project.org/core/dune-localfunctions.git
-  - duneci-install-module https://gitlab.dune-project.org/core/dune-istl.git
-  - duneci-install-module https://gitlab.dune-project.org/core/dune-grid.git
-  - duneci-install-module https://gitlab.dune-project.org/staging/dune-typetree.git
-  - duneci-install-module https://gitlab.dune-project.org/staging/dune-functions.git
-  - duneci-install-module https://gitlab.dune-project.org/staging/dune-uggrid.git
-  - duneci-install-module https://gitlab.dune-project.org/extensions/dune-alugrid.git
+  - source ~/toolchain
+  - export CMAKE_FLAGS="-DCMAKE_C_COMPILER='$CC' -DCMAKE_CXX_COMPILER='$CXX'"
 
-debian:10 gcc-8-noassert-17:
-  image: registry.dune-project.org/docker/ci/debian:10
+debian:10 gcc-8-17:
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-gcc-8-17
   script: duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN: gcc-8-noassert-17
 
 debian:10 clang-6-libcpp-17:
-  image: registry.dune-project.org/docker/ci/debian:10
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-clang-6-libcpp-17
   script: duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN: clang-6-libcpp-17
 
 debian:9 gcc-6-14:
-  image: registry.dune-project.org/docker/ci/debian:9
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-9-gcc-6-14
   script: duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN: gcc-6-14
 
-debian:9 clang-3.8-14:
-  image: registry.dune-project.org/docker/ci/debian:9
-  script: duneci-standard-test
-  variables:
-    DUNECI_TOOLCHAIN: clang-3.8-14
-
-ubuntu:16.04 gcc-5-14:
-  image: registry.dune-project.org/docker/ci/ubuntu:16.04
-  script: duneci-standard-test
-  variables:
-    DUNECI_TOOLCHAIN: gcc-5-14
-
 ubuntu:18.04 clang-6-17:
-  image: registry.dune-project.org/docker/ci/ubuntu:18.04
+  image: registry.dune-project.org/docker/ci/dune:2.6-ubuntu-18.04-clang-6-17
   script: duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN: clang-6-17