add parallel execution to CI

.gitlab-ci.yml

 ---
 variables:
+  DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
+  DUNECI_CMAKE_FLAGS: $DUNE_CMAKE_FLAGS
   DUNE_CONTROL_PATH:  /duneci/modules:$CI_PROJECT_DIR
   DUNE_PY_DIR:        /duneci/modules/dune-py
   # adding this actually leads to the test failure seen in the core modules
-  # DUNECI_PARALLEL:    6  # don't know why this is required here but not for core modules - magic?
+  # don't know why this is required here but not for core modules - magic?
+  DUNECI_PARALLEL:      "6"
   PYTHONPATH:         "/duneci/modules/dune-common/build-cmake/python:/duneci/modules/dune-geometry/build-cmake/python:/duneci/modules/dune-grid/build-cmake/python:/duneci/modules/dune-istl/build-cmake/python:/duneci/modules/dune-alugrid/build-cmake/python:/duneci/modules/dune-fem/build-cmake/python:$CI_PROJECT_DIR/build-cmake/python"
 
 before_script:
@@ -11,7 +14,7 @@ before_script:
   - python3 -m venv /duneci/modules/dune-pip
   - source /duneci/modules/dune-pip/bin/activate
   - pip install --upgrade pip
-  - pip install ufl numpy matplotlib mpi4py # portalocker
+  - pip install ufl numpy matplotlib mpi4py portalocker
   - 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-grid.git
@@ -29,8 +32,9 @@ debian-11-gcc-9-17:
     - duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN:  gcc-9-17
-    DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
-    DUNECI_CMAKE_FLAGS: $DUNE_CMAKE_FLAGS
+    # DUNE_LOG_FORMAT:   '%(asctime)s - %(name)s - %(levelname)s - %(message)s'
+    # DUNE_LOG_LEVEL:    DEBUG
+    # MAKEFLAGS:         "--verbose --output-sync=target --print-directory"
 
 ubuntu:18.04--gcc:
   image: duneci/ubuntu:18.04
@@ -41,6 +45,3 @@ ubuntu:18.04--gcc:
     - duneci-standard-test
   variables:
     DUNECI_TOOLCHAIN:  gcc-7-17
-    DUNE_CMAKE_FLAGS:   "-DDUNE_ENABLE_PYTHONBINDINGS=ON -DDUNE_MAX_TEST_CORES=4 -DBUILD_SHARED_LIBS=TRUE -DDUNE_PYTHON_INSTALL_LOCATION=none -DCMAKE_POSITION_INDEPENDENT_CODE=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_LATEX=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Alberta=TRUE -DCMAKE_DISABLE_FIND_PACKAGE_Vc=TRUE -DCMAKE_DISABLE_DOCUMENTATION=TRUE"
-    DUNECI_CMAKE_FLAGS: $DUNE_CMAKE_FLAGS
-