Add density generation to Worlds

Worlds will now randomly choose a density for themselves.
Add Size Constraints Table

Add the size constraints table from GURPS Space p. 85.
Add random diameter generation

Make Worlds generate their diameter randomly, as described in GURPS
Space p. 85.
Add generation of surface gravity

Make worlds generate their surface gravity in G's.
Make World masses

Automatically calculate the masses of Worlds.
Add pressure generation

Make Worlds automatically calculate their atmospheric pressure and its
category as per GURPS Space p. 78.
Add Gas Giant size parameters

Add the following size parameters to GasGiant:

 - Mass and Density
 - Diameter
 - Cloud Top Gravity
Merge branch 'worldsize'

Close #7