save learned mag bias per sensor (Multi-EKF support)

- handle saving the mag bias per sensor (across all estimator instances using that mag) in sensors/vehicle_magnetometer
 - this is now saving back to the actual mag calibration CAL_MAGn_OFF{X,Y,Z}
 - ekf2 reset mag mag bias on any magnetometer or calibration change
 - use Kalman filter scheme to update stored mag bias parameters using all available bias estimates for that sensor

Co-authored-by: Paul Riseborough <gncsolns@gmail.com>

src/modules/commander/mag_calibration.cpp

@@ -903,16 +903,6 @@ calibrate_return mag_calibrate_all(orb_advert_t *mavlink_log_pub, int32_t cal_ma
 		}
 
 		if (param_save) {
-			// reset the learned EKF mag in-flight bias offsets which have been learned for the previous
-			//  sensor calibration and will be invalidated by a new sensor calibration
-			for (int axis = 0; axis < 3; axis++) {
-				char axis_char = 'X' + axis;
-				char str[20] {};
-				sprintf(str, "EKF2_MAGBIAS_%c", axis_char);
-				float offset = 0.f;
-				param_set_no_notification(param_find(str), &offset);
-			}
-
 			param_notify_changes();
 		}