Deprecate (and fix) CartesianPoints

This addresses an oversight from the APE5 coordinate work that @mhvk pointed out in #2712: `CartesianPoints` should have been deprecated in favor of `CartesianRepresentation`.  It can't be removed outright because it was in the 0.3 API, but its role is entirely subsumed by `CartesianRepresentation` (although the API is a bit different).

Incidentally, it's also broken in 0.4, so this also makes some changes to make it work.

It's possible no one out there actually uses `CartesianPoints`, but it's safer to do this just in case, I think.  We'll plan on removing it completely in 1.0.

@embray or @astrofrog, do you object to this going into 0.4 ?  It is technically a bug (in that `CartesianPoints` is broken in the RC), but a bug in a minor (possibly unused) feature.  More important is getting the depreaction warning in now so we can remove in 1.0.
Since this was reschedule for v0.4.1 could you please move this changelog entry to the new section?  Thanks!