Define the Aufg environment using newenvironment allows us to reset
the Verteilung macro after each exercise. Otherwise if an exercise
has does nor redefine the Verteilung as empty it will be printed
with a stale point distribution from a previous exercise.

The Lsg environment now sets the expected points even if it is not rendered.
This allows us to use the automatic point counting also in "schueler" versions.
All exercise points are stored in an exercise-local counter and are
persisted using refstepcounter and an appropriate label.