save learned mag bias per sensor (Multi-EKF support)

- handle saving the mag bias per sensor (across all estimator instances using that mag) in sensors/vehicle_magnetometer
 - this is now saving back to the actual mag calibration CAL_MAGn_OFF{X,Y,Z}
 - ekf2 reset mag mag bias on any magnetometer or calibration change
 - use Kalman filter scheme to update stored mag bias parameters using all available bias estimates for that sensor

Co-authored-by: Paul Riseborough <gncsolns@gmail.com>

src/lib/sensor_calibration/Gyroscope.cpp

@@ -123,6 +123,18 @@ void Gyroscope::SensorCorrectionsUpdate(bool force)
 	}
 }
 
+bool Gyroscope::set_offset(const Vector3f &offset)
+{
+	if (Vector3f(_offset - offset).longerThan(0.001f)) {
+		_offset = offset;
+
+		_calibration_count++;
+		return true;
+	}
+
+	return false;
+}
+
 void Gyroscope::set_rotation(Rotation rotation)
 {
 	_rotation_enum = rotation;
@@ -183,19 +195,8 @@ void Gyroscope::ParametersUpdate()
 			_priority = new_priority;
 		}
 
-		bool calibration_changed = false;
-
 		// CAL_GYROx_OFF{X,Y,Z}
-		const Vector3f offset = GetCalibrationParamsVector3f(SensorString(), "OFF", _calibration_index);
-
-		if (Vector3f(_offset - offset).norm_squared() > 0.001f * 0.001f) {
-			calibration_changed = true;
-			_offset = offset;
-		}
-
-		if (calibration_changed) {
-			_calibration_count++;
-		}
+		set_offset(GetCalibrationParamsVector3f(SensorString(), "OFF", _calibration_index));
 
 	} else {
 		Reset();