EKF: Improve initialisation of quaternion covariances

Convert uncertainty in initial rotate vector into quaternion covariances using symbolic toolbox derived expressions.
Enable setting of initial angle uncertainty via a parameter

EKF/ekf.h

@@ -366,4 +366,7 @@ private:
 	// rotate quaternion covariances into variances for an equivalent rotation vector
 	Vector3f calcRotVecVariances();
 
+	// initialise the quaternion covariances using rotation vector variances
+	void initialiseQuatCovariances(Vector3f &rot_vec_var);
+
 };