AP_Compass: Reorder backend code
AP_InertialSensor: MPU6000 fix rebase leftover
AP_Compass_Backend: Add _update_sum method

This method updates the _sum, _count and
_last_timestamp variables in the frontend.
AP_Compass: Backend draft
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.
Minor fix in the Compass frontend