Add Temperature Factors Table

Add the temperature factors table from GURPS Space p. 84. The table is
not entirely complete, the entries for Garden and Ocean worlds have
been omitted and will have to be calculated by the world itself.
Add lookup function for the climate type

The lookup function WorldClimate takes one argument, the average suface
temperature in Kelvins and returns the world climate type (GURPS Space
p. 83).
Average surface temperature and climate type

Make worlds generate an average surface temperature and the
corresponding climate type.
Merge branch 'climate'

Conflicts:
	gurps/space/orbitcontents.py: The printinfo has been adjusted to
		reflect previous changes in master of printinfo behaviour.

Close #6