Flare: Use positive pitch during flaring. Relative alt: Use laser altitu...

...de to decide if in range, not relative altitude estimatie which could be off.

@thomasgubler Could you review and comment and/or merge?

This is test-flown yesterday and the pitch set point seems to make sense. Note that I set the flare altitude to 3 meters (probably still can do down there).
this increases the risk of stalls, which are worse than a *slight* nose down landing (which is of course not what we want in the nominal case as well...). However, note that the tecs will anyway pitch up (assuming the speed is correct) due to the flare curve it tries to follow.