Bug fix: Coord.offset was not offsetting proper motion.
The new code uses a rotation matrix instead of spherical coordinates, which slows it down a bit.
This fixed one of the problems with comparing coordinate conversions against the TCC:
the delta orientation was off. It is still off by 0.001 degrees in places, which seems a lot
but is probably tolerable. Still...I would like to know why this happens. It may be due to
handling proper motion differently (by not removing it initially in the new code), though that seems unlikely.
Worse, the scale factor is quite different between the new and old code; sometimes the TCC
reports shrinking when the new code reports expanding.