Rename pointOfContact role co-author to coAuthor

[balazsdukai]

The point of contact roles are in camelCase, except co-author which is kebab-case (https://www.cityjson.org/specs/2.0.1/#pointofcontact).
Could we rename co-author to coAuthor to unify the case convention?

Although, I think this would be a breaking change. Maybe not worth it. What do you think @hugoledoux ?

[hugoledoux]

breaking change, let's leave it that way

(TIL about kebab-case, thanks)

[balazsdukai]

Ok. I'm going to leave this open then, as a reminder for when (if) we have a next major version.