Add system to the world.
        All systems will be processed on World.process()
        system is of type System

def add_system(self, system):
        '''
        Add system to the world.
        All systems will be processed on World.process()
        system is of type System
        '''
        if system not in self._systems:
            system.set_world(self)
            self._systems.append(system)
        else:
            raise DuplicateSystemError(system)