Catch sonar measurements from surrounding obstacles

[friether]

Currently, walls closer than ground, are considered the "new ground" - which, e.g. results in the drone "climbing up" near-by walls. Possible remedies: Check if jumps of the current sonar-based altitude estimate are realistic/valid. If not, switch to pressure-based estimate and possibly back to the sonar-based one once that is close to the pressure-based one again.

[eric-wieser]

Alternatively, take the jump as a new reference, and then it could deal with flying over tables