Example of plotting coordinates on an all-sky map (final pull request)

This should be the final pull request as discussed in issue #2726. There were some problems with my repo, so I had to re-create my branch and so github doesn't allow me to update the original pull request.
Others may disagree, but I think this paragraph should be removed.  The whole point of `Angle` objects is to make it so users *don't* have to do this, but rather can just use `wrap_at`.  I think this then just ends up a distraction.