EKF: Enable GPS flight without magnetometer (#770)

* EKF: Enable GPS flight without magnetometer

Enables takeoff in a non-GPS flight mode with mag fusion type set to MAG_FUSE_TYPE_NONE. After sufficient movement the EKF will reset the yaw tot he EKF-GSF estimate. After that GPS fusion will commence.

* EKF: Fix unconstrained yaw and yaw variance growth when on ground

* EKF: Ensure first yaw alignment can't be blocked

* EKF: Increase yaw variance limit allowed for alignment

Flight test data indicates that an uncertainty value of 15 deg still provides a reliable yaw estimate and enables an earlier alignment/reset if required.

* EKF: Remove unexecutable code

* EKF: Restructure heading fusion

* EKF: parameterise quarter variance check and retune default value

* EKF: Pass by reference instead of pointer

* EKF: Clarify reset logic

* EKF: Remove incorrect setting of mag field alignment flag

* EKF: Non-functional tidy up

* EKF: Fix non-use of function argument

The updateQuaternion function was using the _heading_innovation class variable instead of setting it using the innovation argument.

* EKF: Fix undefined variable

* EKF: Use single precision atan2

* EKF: remove unnecessary timer reset and unwanted execution of reset function

* EKF: Don't declare a mag fault when non-use is user selected

Doing so produces unnecessary user alerts.

EKF/gps_checks.cpp

@@ -85,7 +85,13 @@ bool Ekf::collect_gps(const gps_message &gps)
 		_mag_strength_gps = 0.01f * get_mag_strength(lat, lon);
 
 		// request a reset of the yaw using the new declination
-		_mag_yaw_reset_req = true;
+		if (_params.mag_fusion_type == MAG_FUSE_TYPE_NONE) {
+			// try to reset the yaw using the EKF-GSF yaw esitimator
+			_do_ekfgsf_yaw_reset = true;
+			_ekfgsf_yaw_reset_time = 0;
+		} else {
+			_mag_yaw_reset_req = true;
+		}
 		// save the horizontal and vertical position uncertainty of the origin
 		_gps_origin_eph = gps.eph;
 		_gps_origin_epv = gps.epv;