Progress simulation with a single step.

        Can not be called when some of the agents have not acted for the
        current step.

def step(self):
        """Progress simulation with a single step.

        Can not be called when some of the agents have not acted for the
        current step.
        """
        assert len(self._agents_to_act) == 0
        self.next()
        while len(self._agents_to_act) > 0:
            self.next()