Splitted gitlab ci files

This MR splits the CI files, so that they can be handled by the institutes themselves.

If merged, you must also set the settings for the custom ci configration paths https://docs.gitlab.com/ee/ci/pipelines/settings.html#custom-ci-configuration-path

(Settings > CI / CD > General pipelines > Custom CI configuration path

must be changed to .gitlab-ci-gfz.yml)