AP_InertialSensor: make HIL timing more accurate

needed for 400Hz replay
AP_NavEKF: improved replay timing
AP_NavEKF : Fix variance constraint bug

Constraining variances to a minimum value of 1e-9 was causing problems
with gyro bias and angular accuracy in noisy GPS environments.
Because the constraint is applied after every covariance prediction
and correction, a lower value of 0 is more appropriate.
AP_NavEKF : Improved heading and magnetic field state initialisation
Rover: disable GPS2 in HIL
Copter: disable GPS2 in HIL
Plane: disable GPS2 in HIL