Intermediate state
First version of "Planet". Not tested yet.

This is the first version in which all steps from the German Traveller
manual are implemented.
Unified the classes according to the Traveller RPG rulebook.

The rulebook does not differentiate between systems and planets.
Therefore the two classes were united into one.
Corrected an error in the determination of gas giant presence.

The probability of gas giants was much lower than according to the rulebook.
This has been fixed now.
Merge branch 'feature/spaceMappingStuff' into develop