Now pv is Ap
Docfix
Asteroid.fluxd: return 0 when diameter is <= 0.
Asteroid Docfix.
Asteroid: subclass SolarSysObject and accept one as  input.
Asteroid: simplify fluxd, use specific dictionaries for ref, therm
Asteroid: remove SpiceObject.__init__ code.
New Comet and Coma classes.  Coma not yet finished.