Applied clippy lints and added it to ci

.gitlab-ci.yml

src/input_types/ilias.rs

src/input_types/ipynb.rs

src/lib.rs

src/output_types/meta.rs

src/output_types/submission_type.rs

src/post_process/test_runner.rs

src/transform/students.rs

src/transform/sub_types.rs

src/transform/submissions.rs