EKF: Add method to enable the IMU bias states to be reset externally

EKF/covariance.cpp

@@ -91,6 +91,9 @@ void Ekf::initialiseCovariance()
 	_prev_dvel_bias_var(1) = P[14][14] = P[13][13];
 	_prev_dvel_bias_var(2) = P[15][15] = P[13][13];
 
+	// record IMU bias state covariance reset time - used to prevent resets being performed too often
+	_last_imu_bias_cov_reset_us = _imu_sample_delayed.time_us;
+
 	// variances for optional states
 
 	// earth frame and body frame magnetic field