'Tick one step. We update callcount (tracks number of calls) as
well as null start_delay (needed in order to correctly
estimate next_call_time at all times).'

def __call__(self):

    self.callcount += 1
    if self.start_delay:
        self.start_delay = None
        self.starttime = self.clock.seconds()
    LoopingCall.__call__(self)
