Combining RATE and HOLD PIDs into one

- attitude difference is equivalent to the integration of the rate error.
- reseting to new attitude simply resets the error to zero
- RATE mode should have low PID I term, HOLD mode should have high PID I term (and lower P term)

- also made some improvements to the DELTA/VTAIL mixing for 125% throw. No more multiplications, only shifts and additions
Merge branch 'HH' of https://github.com/noobee/FlightStab into HH

Conflicts:
	FlightStab/FlightStab.ino
Revised PID engine and setup to combine RATE and HOLD.

integrating rate over fixed time steps derives attitude, so the sum_err in the pid state with rate as the control variable represents the amount of attitude change since it was last set to zero. multiply that by the I parameter derives the correction due to attitude change. reseting relative attitude simply clears sum_err to zero.

- new compute_pid(), separate parameters from state.
- 2 sets for pid parameters: RATE and HOLD
- RATE is PD
- HOLD is PID with reduced P (and also D for now)
- reset attitude in HOLD mode by clearing sum_err in pid state to zero

TODOs:
- re-evaluate the PID relative weight shifts, i think PID_KI_SHIFT needs to be lower (from 6 to 4)
- re-evaluate the D parameter in HOLD pid