Fix 'kink' in change in lift due to ground effect tables for C172x an…

[seanmcleod]

…d B737

[bcoconni]

Quoting @seanmcleod from the ML

I glanced at a number of other models in the JSBSim distribution and a number of them didn’t have the ground effect modeled at all, and others like the F-15 and F-16 had curves with different data, i.e. they hadn’t been a simple cut-paste of the C172x or B737 model’s data for ground effect.

Thanks Sean.

[bcoconni]

Just for the record, we are using the same correction than the c172p team PR #922

[agodemar]

Well done.