Flesh out World and AsteroidBelt classes some

Add a type() method to all OrbitContent subclasses to ensure uniform
response modes.

Also add a __repr__ method for uniform representations.

Flesh out the World class some by adding a modified initializer method
that takes the size class of the Terrestrial Planet as an additional
argument.
Add filling of orbits with orbit contents

The PlanetSystem will now automatically populate orbits according to
GURPS Space p. 110.