AP_NavEKF3: document provenance of tilt error variance equations

This is also from the older version of the generator.

Note that as documented, some of the equations have been removed and
rearranged slightly as it is assumed their terms are zero. Additionally,
the result is taken as the sum of the diagonal entries of the matrix.

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -2122,7 +2122,7 @@ void NavEKF3_core::calcTiltErrorVariance()
     const ftype &q2 = stateStruct.quat[2];
     const ftype &q3 = stateStruct.quat[3];
 
-    // equations generated by quaternion_error_propagation(): in AP_NavEKF3/derivation/main.py
+    // equations generated by quaternion_error_propagation(): in derivation/generate_2.py
     // only diagonals have been used
     // dq0 ... dq3  terms have been zeroed
     const ftype PS1 = q0*q1 + q2*q3;

libraries/AP_NavEKF3/derivation/generate_1.py

@@ -678,8 +678,8 @@ def generate_code():
 
 
     # derive autocode for other methods
-    print('Computing tilt error covariance matrix ...')
-    quaternion_error_propagation()
+    # print('Computing tilt error covariance matrix ...')
+    # quaternion_error_propagation()
     # print('Generating heading observation code ...')
     # yaw_observation(P,state,R_to_earth)
     print('Generating gps heading observation code ...')

libraries/AP_NavEKF3/derivation/generate_2.py

@@ -665,8 +665,8 @@ def generate_code():
 
 
     # derive autocode for other methods
-    # print('Computing tilt error covariance matrix ...')
-    # quaternion_error_propagation()
+    print('Computing tilt error covariance matrix ...')
+    quaternion_error_propagation()
     print('Generating heading observation code ...')
     yaw_observation(P,state,R_to_earth)
     # print('Generating gps heading observation code ...')