ArduPPM: Enhanced the switchover and the ppm generator for clean ppm output.

When both inputs were invalid, the ppm generator was starting immediately sometimes causing a frame with more channels. A timer has been added to start the generator after 30 ms if both inputs are invalid instead of firing it immediately.

A force output pin action has been added to avoid polarity inversion when a PPM input was disappearing in a low level state (for a negative ppm frame).

The ppm generator is now stopping on a frame end to avoid generating a corrupted frame.

The switchover has been modified : the ppm validity flag is not used anymore for direct control of the PPM output. A new ppm live ready flag has been created to separate validity and live ready status. This was needed for correct frame starts at the PPM output.

Some cleaning has been done in the watchdog interrupt to simplify logic.

A ppm_first_format_confirmed flag has been added to avoid a ppm generator start before at least one ppm input format has been confirmed. This will keep the  PPM output silent until a ppm input signal has been detected and its format confirmed.

Flowcharts updated to follow beta 5.