'Stop the I/O loop.
If the event loop is not currently running, the next call to `start()`
will return immediately.
To use asynchronous methods from otherwise-synchronous code (such as
unit tests), you can start and stop the event loop like this::
ioloop = IOLoop()
async_method(ioloop=ioloop, callback=ioloop.stop)
ioloop.start()
``ioloop.start()`` will return after ``async_method`` has run
its callback, whether that callback was invoked before or
after ``ioloop.start``.
Note that even after `stop` has been called, the `IOLoop` is not
completely stopped until `IOLoop.start` has also returned.
Some work that was scheduled before the call to `stop` may still
be run before the `IOLoop` shuts down.'

def stop(self):

    raise NotImplementedError()
