EKF2: fix offset from height reference after resets

[bresch]

Solved Problem

When several height sensors are failing at the same time, the EKF resets to the first available measurement. If the height is reset to a secondary height source, this leads to a bias buildup of the height reference.

Solution

Only allow the primary height source to perform a height reset. A secondary height source can only stop and restart (and automatically adjust its own bias to match the current height estimate).

Test coverage

covered by new unit tests

[dagar]

Looks good, but shouldn't we also simplify the reset for each case? We no longer have to fumble with the bias.

PX4-Autopilot/src/modules/ekf2/EKF/aid_sources/external_vision/ev_height_control.cpp

Lines 149 to 150 in 50a823d

	resetVerticalPositionTo(measurement - bias_est.getBias(), measurement_var);
	bias_est.setBias(-_state.pos(2) + measurement);

[bresch]

Looks good, but shouldn't we also simplify the reset for each case? We no longer have to fumble with the bias.

Yes, we can probably simplify the logic a bit, I'll do that in a follow-up PR. That one is just an immediate fix.