pwm: Fix and rework

pwm: Fix and rework

Reworked in the algorithm:
 - Fixed the polarity setting
 - Taken in consideration the first transition
 - Using the 'None' state instead of -1 and 0 value
 - Simplify the algorithm and remove useless branches and variables
 - Avoid re-calculating the same thing more than once
 - Renamed a few variables for a better understanding
 - Duty cycle precision changed to floating value

Otherwise:
 - Added a meta OUTPUT for the duty cycle average
 - Renamed the polarity option:
	'polarity', 'active low/high' are well-understood terms.
 - Added comments

Signed-off-by: Sebastien Bourdelin <redacted>