Determine atmospheric mass

Make Worlds determine their own atmospheric mass.
Add Marginal Atmosphere Roll Table

Add a table to look up marginal atmospheres from a 3d roll (GURPS Space
p. 81).
Determine atmospheric composition

Make worlds determine their atmospheric composition, and whether they
have a marginal atmosphere or not.
Add a table for abbreviations of atm. comp.

For filling a table with information (later on in the TeX'd output) it
is beneficial to work with abbreviations. These are stored in the dict
AtmCompAbbr.
Make planets print out atmosphere information

Planets will now print out the atmospheric composition and eventual
marginal atmospheric conditions in their printinfo() method.
Merge branch 'atmosphere'

Close #4