ekf2: centralize mag covariance reset (#693)

* ekf2: centralize mag covariance reset. A complete and clean reset of the
mag states covariances is now performed through the "resetMagCov"
function only. This avoid having slight differences of implementations
across the code.

* Ekf: reset quat cov with initial uncertainty instead of zero

EKF/ekf.h

@@ -760,9 +760,13 @@ private:
 	// do not call before quaternion states are initialised
 	void initialiseQuatCovariances(Vector3f &rot_vec_var);
 
-	// perform a limited reset of the magnetic field state covariances
+	// perform a limited reset of the magnetic field related state covariances
 	void resetMagRelatedCovariances();
 
+	void resetQuatCov();
+	void zeroQuatCov();
+	void resetMagCov();
+
 	// perform a limited reset of the wind state covariances
 	void resetWindCovariance();