AP_NavEKF: Reduce time to switch after an IMU fault

This revised threshold value is still double the maximum that has been observed in flight logs so far with healthy sensors
The previous value was too slow to switch for sudden IMU gyro faults
We can afford an ocasional false trigger becasue the front end will only select another instance if it is healthy and has lower errors

libraries/AP_NavEKF2/AP_NavEKF2_Outputs.cpp

@@ -57,7 +57,7 @@ float NavEKF2_core::faultScore(void) const
         score += hgtTestRatio-1.0f;
     }
     // If the tilt error is excessive this adds to the score
-    const float tiltErrThreshold = 0.1f;
+    const float tiltErrThreshold = 0.05f;
     if (tiltAlignComplete && yawAlignComplete && tiltErrFilt > tiltErrThreshold) {
         score += tiltErrFilt / tiltErrThreshold;
     }