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/EKF/bias_estimator.cpp

@@ -97,7 +97,7 @@ inline bool BiasEstimator::isTestRatioPassing(const float innov_test_ratio) cons
 
 inline void BiasEstimator::updateState(const float K, const float innov)
 {
-	_state = math::constrain(_state + K * innov, -_state_max, _state_max);
+	_state = _state + K * innov;
 }
 
 inline void BiasEstimator::updateStateCovariance(const float K)