Check if biases states in Kalman filter should be error states or direct states

[rodralez]

Check in Groves' book if biases states in Kalman filter are error states or direct states. Correct the code according.

[rodralez]

It is checked that inertial biases states are error states in Kalman filter. They are the errors between both truth and noisy inertial measurements, as shown in Eq. 4.18, Groves' book.

Modified files are:

1. ins_gnss.m
2. ins_gnss_mag.m
3. ins_visual.m
4. ins_visual_gnss.m