linting and code formatting

Since @k204230 mentioned the code formatting topic in !41 (merged) ...:

I would be grateful for some linting and code formatting in the CI, e.g., we could add a .pre-commit-config.yaml. I had good experience with pre-commit, however the CI would have to enforce this. I am not sure how to add a pre-commit integration to a closed gitlab repo. I think this could somehow be done manually in the .gitlab-ci.yaml. There is an example here that simply installs pre-commit during CI and runs it....