Reformat retrieval experiment, move both classes into main app

This MR first uses ruff format; ruff check --fix to reformat the retrieval.py experiment.

After that, it moves both classes defined in the experiment file into the main application.

This is a preparation for #31 (closed).

Running the code

The experiment now likely needs to be run using the command:

python3 -m experiments.retrieval

To the reviewer

I would recommend reviewing this commit by commit, i.e., going to the "Commits" list and clicking on the oldest linked commit. This will open the following view: !16 (4cfffa7b)

There, you can review the changes in the individual files, mark them as "Viewed" and then go to the next files.

This commit only contains machine-generated changes, so the code should still be correct, however, a small smoke test certainly won't hurt.