RFC-353 by replacing all use of Coord and its subclasses with SpherePoint.
Things to keep in mind:
- afw will no longer support coordinate conversions. We do not rely on this now (except for a very small number of cases of use of FK5 J2000 instead if ICRS that is likely a mistake).
- SpherePoint and Coord have slightly different APIs (for instance SpherePoint is immutable) so some usage will need to change.
- Coord has some methods that SpherePoint does not. These may have to be added to SpherePoint or implemented as free functions.
This would also be a good time to eliminate the SpherePoint(double raDecRad) constructor. This was added to support Transform but turned out to be unnecessary.