RFC-131: Make afw::geom::Coord pure virtual (so it cannot be instantiated) and to add a new class afw::geom::SphPoint that represents a spherical point.
Coord should contain a SphPoint rather than inherit from it. This allows Coord.offset and Coord.rotate to take a SphPoint argument, without the confusion of allowing the user to provide a Coord and wondering what is done with the coordinate system and epoch information.
When implementing, look for opportunities to integrate with sphgeom package, such as borrowing SphPoint from there.
Also be careful to integrate with the upcoming WCS overhaul.
Only support distance if this makes the code simpler in some way (such as being provided by code borrowed from elsewhere). It is explicitly not a requirement.