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.