WIP [ci] add trigger for lecture build

Introduces a first pipeline to Dumux, which simply consists of a trigger to build dumux-lecture after a commit has been made to the master branch.

WPI because it depends on ci integration in dumux-lecture, currently assigned to @emmert.

TODO:

• after merging in dumux-lecture, modify the lecture branch of the trigger to master
• before merge, change rule to $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

.gitlab-ci.yml

+stages:
+    - trigger downstream
+
+# trigger lecture pipeline after commits to master
+trigger lecture:
+    stage: trigger downstream
+    # TODO: Replace by general rules
+    rules:
+      - if: $CI_COMMIT_BRANCH == "feature/ci-trigger-lecture-build"
+    trigger:
+      project: dumux-repositories/dumux-lecture
+      # TODO:##  replace by master
+      branch: feature/test-dumux-trigger
+      strategy: depend
+    variables:
+      DUMUX_BRANCH_NAME: $CI_COMMIT_BRANCH