Stop optimization process once maximum minutes have elapsed.

def _stop_by_max_time_mins(self):
        """Stop optimization process once maximum minutes have elapsed."""
        if self.max_time_mins:
            total_mins_elapsed = (datetime.now() - self._start_datetime).total_seconds() / 60.
            if total_mins_elapsed >= self.max_time_mins:
                raise KeyboardInterrupt('{} minutes have elapsed. TPOT will close down.'.format(total_mins_elapsed))