AP_Compass: Add back/front-end separation
AP_Compass_HMC5843: Adjust to new backend
AP_Compass_HIL: Adjust to new backend
Copter: Adapt compass to the new interface

Compass has been separated in a back/front-end
interface. This commit allows the code to use
this new interface.
AP_InertialSensor: MPU6000 fix rebase leftover