Initial PlanetSystem class

Add the PlanetSystem class with one method:

 - allowedorbit(self, testorbit): Check whether the supplied orbit is
   valid.
Add barebones GURPS_OrbitContents

Add a skeleton file that outlines the contents of GURPS_OrbitContents.

This file will then be fleshed out with the class methods. A good way
to implement expanding of the __init__ method is needed.
Add blackbody temperature to OrbitContent

Add the capability of calculating the blackbody temperature for the
OrbitContent class.

Change the names of the classes somewhat and make GasGiant a child of
OrbitContent instead of World.
Add GasGiant methods makesize and printinfo

Roll for random size generation of the GasGiant.
Complete placement of first gas giant

Complete the placement of the first gas giant, according to the gas
giant arrangement.

Add a class method "printinfo" to PlanetSystem.
Add Orbital Spacing Table

Add the Orbital Spacing Table (GURPS Space p. 109). The index is the
roll, since 3d are rolled without any modifiers.
Add generation of orbits

Add the generation of orbits in PlanetSystem. If there is no gas giant,
orbit generation proceeds outwards from the inner limit. If there is a
gas giant, orbit generation proceeds inwards and outwards from the
orbit of the first gas giant.
Remove debug information

Remove print() statements that were used for debugging purposes.
Add automatic generation a PlanetSystem instance

Add the automatic generation of a PlanetSystem instance for each star
in the StarSystem.
Fix bug in orbit generation

Fix a bug that occurs in outward orbit generation when starting from
a very small orbital radius.