End the simulation and destroy the current simulation environment.

def end(self, folder=None):
        """End the simulation and destroy the current simulation environment.
        """
        ret = self.env.destroy(folder=folder)
        self._end_time = time.time()
        self._log(logging.DEBUG, "Simulation run with {} steps took {:.3f}s to"
                  " complete, while actual processing time was {:.3f}s."
                  .format(self.age, self._end_time - self._start_time,
                          self._processing_time))
        return ret