AP_NavEKF2: Improve timing accuracy

This parameter is a compromise between numerical accuracy of the covariance prediction and sensor timing jitter
Further testing has shown that doing covariance prediction and sensor fusion every 10msec has no observable effect on fusion health and reduces timing hitter noise on magnetometer observations during high rate maneovures

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -402,7 +402,7 @@ NavEKF2::NavEKF2(const AP_AHRS *ahrs, AP_Baro &baro, const RangeFinder &rng) :
     gndEffectTimeout_ms(1000),      // time in msec that baro ground effect compensation will timeout after initiation
     gndEffectBaroScaler(4.0f),      // scaler applied to the barometer observation variance when operating in ground effect
     gndGradientSigma(2),            // RMS terrain gradient percentage assumed by the terrain height estimation
-    fusionTimeStep_ms(20)           // The nominal number of msec between covariance prediction and fusion operations
+    fusionTimeStep_ms(10)           // The nominal number of msec between covariance prediction and fusion operations
 {
     AP_Param::setup_object_defaults(this, var_info);
 }