AP_NavEKF3: resetHeightDatum stricter early return

[gdavud]

Summary

Attempts to fix issue #32720 with altitude spikes on termination (or disarm). AP_NavEKF3 lib treats all disarmed states as definetly on-ground. During flight termination, the vehicle can be airborne while disarmed which can trigger a height origin reset, causing an altitude spike.

Classification & Testing (check all that apply and add your own)

• [ x] Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• [ x] Logs available on request

Description

NavEKF3_core::detectFlight() marks NavEKF3_core::onGround as true, this allows NavEKF3_core::resetHeightDatum() to run which may update NavEKF3_core::ekfGpsRefHgt which in turn may update the vehicle's local_origin in NavEKF3_core::getPosD() causing discontinuity in vertical position.

Function resetHeightDatum() returns false immediately if onGround is false. During the untermination test, onGround is true when the vehicle terminates (even tho we are still airborne), which aligns with the spike from the original issue.

This pr prevents NavEKF3_core::resetHeightDatum from executing if vehicle is disarmed while airborne.
Untermination test running on this patch:

---

Some logs that explain the original issue:

AT-0001.8: AP: Throttle disarmed
arduplane: posD=-52.596778, onGround=0, inFlight=1, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=1
arduplane: posD=-52.755922, onGround=1, inFlight=0, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=0
...
AT-0001.8: Waiting for disarm (0.00s so far of allowed 30.00)
...
AT-0001.8: DISARMED after 0.00 seconds (allowed=30.00)
AT-0001.8: Waiting for text : terminating due to gcs request
AT-0001.8:   statustext got=(Terminating due to GCS request) want=(Terminating due to GCS request)
AT-0001.8: Found expected text in collection: terminating due to gcs request
...
AT-0001.8: Sending COMMAND_INT to (1,1) (MAV_CMD_DO_FLIGHTTERMINATION=185) (p1=0.000000 p2=0.000000 p3=0.000000 p4=0.000000 p5=0 p6=0  p7=0.000000 f=5)
...
AT-0001.8: ACK received: COMMAND_ACK {command : 185, result : 0, progress : 0, result_param2 : 0, target_system : 250, target_component : 250} (0.000000s)
AT-0001.8: Waiting for DISARM
AT-0001.8: AP: Aborting termination due to GCS request
...
AT-0001.8: Waiting for disarm (0.00s so far of allowed 30.00)
arduplane: posD=-56.713185, onGround=1, inFlight=0, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=0
...
arduplane: posD=-56.213194, onGround=1, inFlight=0, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=0
arduplane: posD=-112.811547, onGround=1, inFlight=0, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=0
arduplane: posD=-108.178366, onGround=1, inFlight=0, highAirSpd=1, highGndSpd=1, largeHgtChange=1, motorsArmed=0
...

[peterbarker]

Can you see this change is in direct contravention of the comment immediate after it?