Starts the timer.

def start(self) -> None:
        """Starts the timer."""
        # Looking up the IOLoop here allows to first instantiate the
        # PeriodicCallback in another thread, then start it using
        # IOLoop.add_callback().
        self.io_loop = IOLoop.current()
        self._running = True
        self._next_timeout = self.io_loop.time()
        self._schedule_next()