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/terrain_estimator.cpp

@@ -88,6 +88,11 @@ bool Ekf::initHagl()
 
 void Ekf::runTerrainEstimator()
 {
+	// If we are on ground, store the local position and time to use as a reference
+	if (!_control_status.flags.in_air) {
+		_last_on_ground_posD = _state.pos(2);
+	}
+
 	// Perform initialisation check and
 	// on ground, continuously reset the terrain estimator
 	if (!_terrain_initialised || !_control_status.flags.in_air) {