Check what happens when sharing PWM channel pins

[martinohanlon]

We should investigate what happens when PWM channel pins are shared.

PWMOutputDevice will check and only allow 1 PWM device per PWM channel.

e.g. if you try and create PWMLED objects on pin 2 and pin 18 you will be presented with an error. This is because they share the same channel and the output on 1 pin will interfere with the output on another.

However... if you create a PWMLED on pin 2 and a DigitalLED on pin 18 you wont be presented with an error. This is based on the assumption that only 1 pin is using the PWM channel so all is fine... However I am not sure that this is true and we should check. I am also not sure what would happen if you had an PWMOutputDevice on pin 2 and an InputDevice on pin 18!