ArduPPM : Redundancy mode

Work in progress on a new Redundancy dual PPM sum mode for PPM encoder.

- New library PPM_Encoder_v3.h and new manual manual_v3.txt

- New format conversion capability between input and output PPM frame timings and channel count.

This will be experimental until heavily tested. The main goal is to allow the use of low cost satellite receivers in a high safety setup, and allow a new teacher / student RC mode without link between the two pilot transmitters.