'Install a timer, which calls fun after t milliseconds.
Arguments:
fun -- a function with no arguments.
t -- a number >= 0
Example (for a TurtleScreen instance named screen):
>>> running = True
>>> def f():
if running:
fd(50)
lt(60)
screen.ontimer(f, 250)
>>> f()   ### makes the turtle marching around
>>> running = False'

def ontimer(self, fun, t=0):

    self._ontimer(fun, t)
