#2919 FGElev does not take airport elevation into account WS3.0

[Rick Gruber-Riemer]

Given that airports no longer are baked into the scenery, FGElev only takes the underlying scenery's elevation into account. However, this can lead to many metres of difference in elevation, when a building is placed in an area, where there is an airport layout.
There are at least 2 solutions:
* An additional parameter pointing to a scenery folder with airports - and fgelev makes the necessary
* If only osm2city will ever use this, then point vanosten to the code in SimGear and then vanosten will bake it into osm2city.

I am not sure whether this affects the UFO and Nasal space - most probably not.