IMU Errror jacobian

[cumtchenchang]

Hi:
In IMUError.cpp， const double sigma2_p = 0.5 * dt * dt * sigma2_v;
is const double sigma2_p = 0.25 * dt * dt * sigma2_v?
Thank you

[JzHuai0108]

The contribution of accelerometer noise (of continuous time noise magnitude sigma_a_c) on the covariance of the position from t1 to t2 can be computed by the double integral.
\int_t1^t2 sigma_a_c^2 (\int_\tau^t2 du)^2 d\tau = sigma_a_c^2 * 0.5 * (t2- t1)^3. (用梯形法算的)。 如果精确的算是 sigma_a_c^2 * 0.33 * (t2- t1)^3
For more info, see my thesis eq (4.31) on page 102 and eq (A.25) on page 194.