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/common.h

@@ -197,9 +197,10 @@ struct parameters {
 	float terrain_p_noise;		// process noise for terrain offset (m/sec)
 	float terrain_gradient;		// gradient of terrain used to estimate process noise due to changing position (m/m)
 
-	// initial switch on bias uncertainty
+	// initialisation errors
 	float switch_on_gyro_bias;	// 1-sigma gyro bias uncertainty at switch on (rad/sec)
 	float switch_on_accel_bias;	// 1-sigma accelerometer bias uncertainty at switch on (m/s**2)
+	float initial_tilt_err;		// 1-sigma tilt error after initial alignment using gravity vector (rad)
 
 	// position and velocity fusion
 	float gps_vel_noise;		// observation noise for gps velocity fusion (m/sec)
@@ -292,9 +293,10 @@ struct parameters {
 		terrain_p_noise = 5.0f;
 		terrain_gradient = 0.5f;
 
-		// initial switch on bias uncertainty
+		// initialisation errors
 		switch_on_gyro_bias = 0.1f;
 		switch_on_accel_bias = 0.2f;
+		initial_tilt_err = 0.1f;
 
 		// position and velocity fusion
 		gps_vel_noise = 5.0e-1f;