Add: PWM support relay mode

[RaulTrombin]

Check if PWM crate support relay mode like ardupilot

[patrickelectric]

relay mode can be accomplish using a 0xffff as channel value

[RaulTrombin]

Actually the 409"6" value isn't acepted by the pca library method.

Cause it sets the OFF counter, not the dutycicle directly, you have ON & OFF.

For this, it's possible to use the full_on/full_off methods.

Already tested. Just need to clean the code

#68

[RaulTrombin]

We added this function with previous PR.