ekf2: enable multiple height sources fusion

Instead of having a single height source fused into the EKF and the
other ones "waiting" for a failure or the primary sensor, fuse all
sources in EKF2 at the same time. To prevent the sources from fighting against each
other, the "primary" source is set as reference and the other ones are
running a bias estimator in order to make all the secondary height
sources converge to the primary one.

If the reference isn't available, another one is automatically selected
from a priority list. This secondary reference keeps its current bias
estimate but stops updating it in order to be the new reference as close
as possible to the primary one.

src/modules/ekf2/ekf2_params.c

@@ -608,22 +608,31 @@ PARAM_DEFINE_FLOAT(EKF2_TAS_GATE, 3.0f);
  * 5 : Set to true to enable multi-rotor drag specific force fusion
  * 6 : set to true if the EV observations are in a non NED reference frame and need to be rotated before being used
  * 7 : Set to true to enable GPS yaw fusion. Cannot be used if bit position 4 is true.
+ * 8 : Set to true to enable vision velocity fusion
+ * 9 : Set to true to enable barometer height fusion.
+ * 10 : Set to true to enable GPS height fusion.
+ * 11 : Set to true to enable range finder height fusion.
+ * 12 : Set to true to enable vision height fusion.
  *
  * @group EKF2
  * @min 0
- * @max 511
- * @bit 0 use GPS
- * @bit 1 use optical flow
+ * @max 8091
+ * @bit 0 position: GPS
+ * @bit 1 velocity: optical flow
  * @bit 2 inhibit IMU bias estimation
- * @bit 3 vision position fusion
- * @bit 4 vision yaw fusion
- * @bit 5 multi-rotor drag fusion
+ * @bit 3 position: vision
+ * @bit 4 yaw: vision
+ * @bit 5 wind: multi-rotor drag fusion
  * @bit 6 rotate external vision
- * @bit 7 GPS yaw fusion
- * @bit 8 vision velocity fusion
+ * @bit 7 yaw: GPS
+ * @bit 8 velocity: vision
+ * @bit 9 height: barometer
+ * @bit 10 height: GPS
+ * @bit 11 height: range finder
+ * @bit 12 height: vision
  * @reboot_required true
  */
-PARAM_DEFINE_INT32(EKF2_AID_MASK, 1);
+PARAM_DEFINE_INT32(EKF2_AID_MASK, 513);
 
 /**
  * Determines the primary source of height data used by the EKF.