Fixed some problems with class Quadtree.

Default copy constructor and assingment operator caused runtime error.
It didn't work because of dynamic memory copied. Both copy constructor
and assingment operator is now implemented to fix this. Added support
for iterators, i.e. a quadtree can now be iterated without need to copy
the internal container. There was some other minor interface changes
also.
Removed duplicated code.

Commonly used code moved to auxiliary.h. And added a function to
determen if a point lies between two parallel lines.
A Quadtree is now used in class Zombiegame.

All buildings in class Zombiegame is now stored in a quadtree instead of
a vector. Some code was added in order to create the quadtree correctly.