Destroy the environment and the subprocesses.

def destroy(self, folder=None):
        '''Destroy the environment and the subprocesses.
        '''
        ameans = [(0, 0, 0) for _ in range(3)]
        ret = [self.save_info(folder, ameans)]
        aiomas.run(until=self.stop_slaves(folder))
        # Close and join the process pool nicely.
        self._pool.close()
        self._pool.terminate()
        self._pool.join()
        self._env.shutdown()
        return ret