FF: CountdownTimer init use reset instead of add for initial time

[gtosh4]

Following the example & documentation,

timer = core.CountdownTimer(5)

would cause the warning to show

WARNING We strongly recommend you activate the PTB sound engine in PsychoPy prefs as the preferred audio engine. Its timing is vastly superior. Your prefs are currently set to use ['sounddevice', 'PTB', 'pyo', 'pygame'] (in that order).

How I found it (if that's at all useful) is to replace the Clock.add function to raise an exception to get the traceback.

def newAdd(self, x):
    raise Exception("Clock.add is deprecated")

core.Clock.add = newAdd

which resulted in the traceback:

  File "XXX\experiment1.py", line 330, in testF
    timer = core.CountdownTimer(2)
  File "C:\Program Files\PsychoPy\lib\site-packages\psychopy\clock.py", line 226, in __init__
    self.add(start)
  File "XXX\experiment1.py", line 30, in newAdd
    raise Exception("Clock.add is deprecated")

While I was there, I also simplified the CountdownTimer.reset() function a little.