HAL_AVR: changed to 16 byte bulk transfer on SPI0
AP_InertialSensor: try to lower SPI bus speed on errors
AP_InertialSensor: automatically lower bus speed on mpu6k bad reads
AP_Math: fixed zero function for integer vectors
AP_InertialSensor: detect bad MPU6000 SPI transactions and lower bus speed

this uses bad data or bad INT_STATUS values from the MPU6000 to detect
the sensor running too fast and lower bus speed
AP_Baro: start MS5611 at high speed
Rover: report INS health
Copter: report INS health
Plane: report INS and airspeed health
AP_Airspeed: added healthy() API
SITL: added SIM_ACCEL_FAIL option

used to test accelerometer failure in flight
AP_InertialSensor: added healthy check for PX4 and HIL

used to detect bad accels
AP_AHRS: detect and try to cope with bad accels
Plane: removed unused variable