add pre-commit with toxenv

[obestwalter]

Description

Add a lint step that does automatic code formatting via black using the pre-commit framework. There are also a few commented out hooks, that seem sensible to add, but this should be done in several steps. The first step should be adding formatting. As an example I ran tox -e lint and pushed it to a different branch. This is what the formatted code looks like in PR #86

Running tox -e lint on a local machine also installs the pre-commit hook, so that committing code that is not properly formatted or is otherwise flagged (e.g. via flake8) can not be commited.

[isidore]

we want to do this, but are going to wait to do it together with you present

[claremacrae]

I guess when we accept this, we should add a note to the docs somewhere, asking contributors to run tox -e lint to enable the pre-commit hook.

[obestwalter]

Yes, that was what I wanted to talk about then ... if we really want to add this automatic step, which then also needs some explanation in the docs for developers, who might be surprised by this. But we can talk about this next week then,