Position class is now over-complicated, with far too much fiddly branching. Or is it? Things like
TrackingSchedule need specific docmentation concerning which coordinate system the arguments are in, with the result that rotation-by-colatitude happens in multiple places (and I always get the signs wrong first time). I can't help feeling these should be in a single place, but every time I conclude that single place is 'in
Position', I end up with a mess in there.