ArduPPM: The PPM output polarity stay now constant regardless the input polarity and failsafe.

Solved a ppm output polarity inconsistency when starting the PPM generator :

A force compare match is now used for timer 1 during a ppm pin level init procedure.
"Clear or set on compare match" modes are now used to force the output pin to the right state.
Then the timer compare mode is changed again to "toggle on compare" for normal run.

Added the logic to set output polarity according to _POSITIVE_PPM_FRAME_ #define