Add getters to Star

Add several getX methods to the Star class.
Change initializer of PlanetSystem

Change the init method of PlanetSystem so that it only needs one
argument: the parent star. All values needed for the next steps are
then queried from the parent star.
Pass primary star to init methods of orbit contents

GasGiants, Moons and Planets will now store their primary star. In the
future this will be utilized to check the luminosity of the star, which
in turn may help decide between some world types.
Add World method maketype

The World method maketype() decides with help of size and blackbody
temperature which world type is appropriate for it. Close #3
Make printinfo of PlanetSystem more verbose

Add several new points of information to the printinfo method of a
PlanetSystem. It now outputs more details about each planet, such as
number of moons and moonlets (or number of objects in one of the three
families for gas giants) and world type.
Merge branch 'world-type'