ArduPPM:  Beta 4  - Enhanced the validity checker and added its flowchart documentation.

Beta 4 version update and new compiled binary.

Added a detection based on the frame completed flag.
A counter with a settable detection threshold has been added.
A #define has been added to set it :
FRAME_COUNT_DETECTION_THRESHOLD     2   // Valid frames needed before declaring a PPM input valid

A PPM input is now declared valid if a number of frames (threshold) have been fully validated.

Frame sync, channel sync, channels count, and channels length validity are checked.

The default setting is to wait for 2 valid frames before to declare an input valid.

This should remove any chance to have invalid frames targeting the PPM output with a high confidence level.