EKF: Add method to fuse horizontal magnetometer data

This method is more suitable than a raw heading measurement because it works across a full range of pitch angles.
It has been made the default for ground operation.

EKF/ekf.h

@@ -183,9 +183,12 @@ private:
 	// ekf sequential fusion of magnetometer measurements
 	void fuseMag();
 
-	// fuse magnetometer heading measurement
+	// fuse heading measurement (currently uses estimate from magnetometer)
 	void fuseHeading();
 
+	// fuse projecton of magnetometer onto horizontal plane
+	void fuseMag2D();
+
 	// fuse magnetometer declination measurement
 	void fuseDeclination();