Add representation method for Star

Add a __repr__ method for the Star class, consisting of a tuple
(mass, luminosity, temperature).
Correct error in findsequence

Correct a grave error in findsequence that resulted in far too many
white dwarves.

Also: Remove some unnecessary, out-commented statements.
Add sorting of the stars

Add automatical sorting of the stars according to their mass
(descending). StarSystem.stars will then be a sorted list of Stars.

Also: Change the way printinfo behaves. Call printinfo of each Star in
the list, so that it appears in order (because printinfo is called
after the sorting).