Time subclasses and using them to initialize Times #10654

Time.__new__ has special treatment for Time instances, essentially passing them through with perhaps a scale change. It turned out this included TimeDelta, so that Time(TimeDelta(...)) returned a TimeDelta instance, which was surprising and caused #10651. In #10652, the bug was fixed by restricting the special case to instances of the class proper. But as @taldcroft notes, in general one would like for Time(TimeSubClass(...)) to work and to return a Time instance. It would be good to allow that.

Part of the problem is that TimeDelta should probably not be a subclass of Time - indeed, as noted in #6828 it shares methods such as .now() and .light_travel_time() that really make no sense. In #6828, it was suggested a better scheme might be to have both Time and TimeDelta inherit from a common TimeBase.