Fix race condition when wait_for_event is used at the same time as direction_up etc.

[aonghusonia]

if you already have a callback assigned to one of the directions ot any and you wait_for_event the first press gets ignored because its read by the callback.

wait_for_event now raises an error if there is a joystick callback assigned. Also added a warning to the documentation

[aonghusonia]

This code replicates the issue

from sense_hat import SenseHat
from signal import pause

sense = SenseHat()

def callback(event):
    print("direction_up callback: The joystick was {} {}".format(event.action, event.direction))

sense.stick.direction_up = callback

event = sense.stick.wait_for_event()
print("wait_for_event: The joystick was {} {}".format(event.action, event.direction))

pause()