Sets the tag.
        If the Entity belongs to the world it will check for tag conflicts.

def set_tag(self, tag):
        '''
        Sets the tag.
        If the Entity belongs to the world it will check for tag conflicts.
        '''
        if self._world:
            if self._world.get_entity_by_tag(tag):
                raise NonUniqueTagError(tag)
        self._tag = tag