AP_InertialSensor: added support for multiple accel/gyro devices

this makes it possible to ask for the gyro and accel vectors from
secondary INS devices.
AP_InertialSensor: implement up to two sensors on PX4
DataFlash: added Log_Write_IMU2() for logging 2nd INS sensors
PX4: start all sensor types on PX4

this allows for secondary sensors
Copter: log both primary and secondary INS samples
AP_Math: added is_zero() method to Vector3
AP_InertialSensor: generalise the accel/gyro calibration for N sensors
DataFlash: fixes for INS API change
GCS_MAVLink: update from upstream XML
GCS_MAVLink: re-generate MAVLink headers
AP_Compass: switched to a vector based interface

this is more consistent with other APIs and makes multi-device support
easier
Rover: updates for compass API change
Copter: updates for compass API change
Plane: updates for compass API change
APA_AHRS: update for compass API change
SITL: update for compass API change
Plane: breakup one second loop into 3 pieces

this prevents it not running when gimbal is active
Plane: lower the priority of mount update
DataFlash: log both IMU packets at once
Copter: don't need Log_Write_IMU2() now