Make repository ready for more contributers

We need to make the repository more robust to future development and help new users to contribute following our standard practises. Let's discuss this here and generate some jobs.

Here are some obvious ones

• Create overview "map" of the code to orientate new users
• Add contributors guide
• Add to CI PEP8 checking
• Add to CI checking of "simplicity"

Please feel free to add extra tasks / claim a task.

@colm.talbot @moritz.huebner @paul-lasky