The Body's state at the instant: the Position, speed, Course and Sunlit
state propagate, orbitalSpeed, course and isSunlit answer for its
Elements, to the bit, from one propagation instead of four. Throws a
PropagationError where SGP4 fails at the instant, as propagate does.
A Body's propagation built once from its Elements: the SGP4 record and whether the orbit is Geostationary, worked out when it is built and never again, so each instant asked costs one propagation and the arithmetic round it. A caller that animates a marker builds one per Body per set of Elements and asks it every second.