Creating a pipeline for testing and linting

Now Gitlab CI is being used to run the unittests. The order is the following:

1. Pre-commit runs and if changes are detected it makes a new commit
2. If pre-commit succeeds 3 jobs are started in parallel:
   • 1. Gitlab code quality checks (is allowed to fail)
   • 2. Pylint: This is way stricter than pre-commit!
   • 3. Unittest: testing all our tests
3. Only if Unittests are successful, and we are on the main branch, then we build the documentation
4. Upload the documentation

Doing this, I had to revert to python 3.9 because pytorch-lightning does not provide a python 3.10 image. Also the version is now automatically managed using https://github.com/pypa/setuptools_scm/

Closes #23 (closed) #10 (closed) #9 (closed)