Clarification of code of conduct

[namurphy]

At present there are two parts of the repository that fulfil the role of a code of conduct: vision_statement.md and CODE_OF_CONDUCT.md. The section from the vision statement is adapted from Astropy, and the code of conduct file is adapted from the contributor covenant standard. There may be some inconsistencies between the two, so it would probably be best to merge the two and keep it in one easily found place.

We also need to clarify the procedure for how violations of the code of conduct are dealt with (e.g., who complaints are submitted to, what procedure they undertake, what the consequences will be, etc.).

[namurphy]

Another possibility: Open Code of Conduct