Linting

This MR adds a .pre-commit-config.yaml configuration file for pre-commit. There is also an update on the linting workflow, that now includes checking for flake8 and isort. Running pre-commit should always make sure that linting passes. There is also a .flake8 config files which ignore E501 (maximum line lenght). black can not always fix line lengths, so this is ignored for now. I ran pre-commit run --all-files already... I also fixed the docs, so this MR should close #42 (closed).

closes #42 (closed), #37 (closed)