implement PWM mock functionality

[2bndy5]

PWM is done via pwmio.PWMOut class.

The pulseio module might also be relevant and similarly implemented (at a glance).

[2bndy5]

Given that the duty cycle might be calculated, it might be a good idea to let SetDutyCycle constructor accept a specified range.

Note

Reading the duty cycle may have different behavior depending on which board is used. This implementation detail shall be neglected, unless a better approach is discovered.