precastro.py: Object becomes SiderealObject, an instance of novas_object
Begin implementing EphemObject
Implement EphemObject.ephem() and use in Time.asBJD()
precastro.py: implement EphemObject.astropos()

Note that we're not actually defining the method generically
on CelestialObject. I'll get around to that at some point.
Implement horizon coordinates for EphemObjects.

This means we can find out when sunset is.