MockPin fixups
MockPin fixups

* alter the PinSetInput exception message to match other `Pin` implementations
* constrain the state of MockPin to a `bool`, and the state of MockPWMPin to a `float`
* allow MockPin to have a `None` frequency set, matching the docs http://gpiozero.readthedocs.org/en/latest/api_pins.html#gpiozero.pins.Pin.frequency
lurch/python-gpiozero,waveform80/gpio-zero,MrHarcombe/python-gpiozero,RPi-Distro/python-gpiozero