Compass: set_motor_compensation takes instance as first parameter

Also get_motor_compensation uses primary compass
Compass: get_primary method made public
Compass_VRBrain: get_primary made public
Compass_PX4: get_primary made public
Copter: compass mot configures all compasses
Copter: CLI report_compass prints results for all compasses
Copter: add ekf check of compass

Checks the EKF's compass variance or the inertial navigations x and y
acceleration corrections
Copter: add EKFCHECK_COMPASS parameter
Copter: integrate EKF check

Check runs at 10hz
Log ekf check failures and ekd failsafes
AP_Notify: add EKF failure warning