A quadtree can now be cleared.

Implemented a clear memeber function which remove all objects inside the
quadtree.
Fix time step now increased.

The game will now have more time in every timestep.
class PhysicalEngine is optimized.

PhysicalEngine  is now implemented in physicalengine.cpp. All static
physical objects are now in a quadtree which will increase the speed of
the interactions with static objects and units.
Simplified init code in class ZombieGame.