Add code of conduct
@all I think it would be good to adapt and official code of conduct in the DuMux community and the development of DuMux. This would be a formalization of what we already (hopefully) practiced and tried to communicate to developers. I suggest to add en established form (e.g. the [Contributor Covenant](https://www.contributor-covenant.org/ as suggested in this MR). Small modifications are of course possible (personally I don't think this is necessary).
- Contact method