Since motors with increasing speed due to yaw input seem to generate more thrust that motors that slow lose thrust, thus net thrust goes up, causing copter to climb.
Values are a guesstimate, proven out by test flying. This could probably become a parameter.
1. it's step towards rearchitecting the current code base ahead of a move to RTOS.
2. internally it uses a MatrixTable for the Quad, Octa, OctaQuad, Y6 frames.
3. it implements the missing stability patch for Octa and OctaQuads (still missing for Y6)
Later check-ins will incorporate into the main ArduCopter code.