add covariance matrices index aliases

msg/vehicle_odometry.msg

...odules/attitude_estimator_q/attitude_estimator_q_main.cpp

src/modules/ekf2/ekf2_main.cpp

src/modules/local_position_estimator/sensors/mocap.cpp

src/modules/local_position_estimator/sensors/vision.cpp

.../position_estimator_inav/position_estimator_inav_main.cpp

src/modules/simulator/simulator_mavlink.cpp