Replay: added accel and gyro mask options
Replay: fixed -g option
AP_NavEKF : Add accel aliasing protection
AP_NavEKF : Add div0 protection to the IMU1_weighting calc
AP_NavEKF : Fix bug that prevents Zacc bias state variance being updated
AP_NavEKF : Use average of dual IMU gyro data when available
AP_NavEKF: don't assume the number of gyros == number of accels
AP_NavEKF: cope with compass going offline while in flight
Plane: Added concept design for coupled roll/pitch control