unify Densify trait (adds support for Geodesic, Rhumb)

[michaelkirk]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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Supersedes #1208 Part of #1181

Densify is now generic, and implemented for anything that implements Distance<Point, Point> and Interpolate<Point>.

Specifically that means we continue to support Euclidean and Haversine densification, and now additionally support Geodesic and Rhumb.

[michaelkirk]

Pushed up some missing docs - sorry 🙇