Add precommit hooks

[LauLauThom]

As discussed today, here is a suggestion to use pre-commit to automatically format the code to fulfill flake8 and other code formatting practices.
The precommit-config will pick up the flake8 config from the setup.cfg

I added the instructions how to install precommit and the hooks defined in the configuration file to the contributing.md.
I think this could even be part of the CI/CD but I have never done this myself. Currently this would require contributors to actively do these installation steps (rather straightforward though).

I passed the codebase through the hooks, it only corrected some json and notebook files, which we could also exclude from the hook checks by customizing the rules in the config file if we want.

[simleo]

Thanks @LauLauThom!