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