Add representation for Gas Giant

Add a __repr__ method for the GasGiant class for a representation of
the format "[Size] Gas Giant".
Add a dictionary to keep track of orbit contents

The dictionary has the orbital radius as key (since this is unique) and
contains primarily None as value.

During initialization of the dictionary perform the check for existence
of a first gas giant and place it in there if that is the case.
Make output listing of orbit contents nicer

Sort the orbitcontents dict according to key (orbit radius) and make
a nice, line-broken display of the contents.
Export calculation of size bonus for gas giants

Make a method (PlanetSystem.gasgiantbonus(orbit)) that returns a bool
value for the size bonus of a gas giant (that has to be passed during
initialization).
Automate placement of gas giants

Automate the placement of gas giants according to GURPS Space p. 110.
Make GasGiant silent and identifiable

Remove the automatic printinfo that occurs after initialization of a
GasGiant instance.

Also: Add a method type() that returns a string "Gas Giant". This is
for easier identification of what type of orbit content we have at some
point.