Add resource values to AsteroidBelt

Add implementation of Resource Value Modifier and Overall Value to the
AsteroidBelt class.
Add RVM and Resource Value to Worlds

Terrestrial Worlds now also generate a Resource Value Modifier and the
corresponding Overall Value. (GURPS Space pp. 87 and 121)
Add habitability score to Worlds
Add affinity scores to Worlds and AsteroidBelts
Merge branch 'resources': close #10