EKF: Improve protection against severe IMU accel errors

Use vertical velocity and position innovation failure to detect bad accelerometer data caused by clipping or aliasing which can cause large vertical acceleration errors and loss of height estimation. When bad accel data is detected:

1) Inhibit accelerometer bias learning
2) Force fusion of vertical velocity and height data
3) Increase accelerometer process noise

EKF/common.h

@@ -169,6 +169,11 @@ struct extVisionSample {
 #define RNG_MAX_INTERVAL	2e5
 #define EV_MAX_INTERVAL		2e5
 
+// bad accelerometer detection and mitigation
+#define BADACC_PROBATION	10E6	// Number of usec that accel data declared bad must continuously pass checks to be declared good
+#define BADACC_HGT_RESET	1E6	// Number of usec that accel data must continuously fail checks to trigger a height reset
+#define BADACC_BIAS_PNOISE_MULT 2.0f	// The delta velocity process noise is multiplied by this value when accel data is declared bad
+
 struct parameters {
 	// measurement source control
 	int fusion_mode;		// bitmasked integer that selects which of the GPS and optical flow aiding sources will be used