Fix bug in API function "tiglWingGetSegmentUpperSurfaceAreaTrimmed"

[AntonReiswich]

Bug Description
The API function tiglWingGetSegmentUpperSurfaceAreaTrimmed(...) terminated with return code 1, when passing the valid arguments (tiglHandle, 1, 1, 0, 0, 0, 1, 0.05, 1, 0.05, 0, double*ReturnValue).

Bug Analysis
This bug arises due to the fact that the implementation of the mentioned function contained a cast from TopoDS_Compound to a TopoDS_Face. As the used test data configuration has the property, that both its upper and lower wing consists of more than one face, respectively, arising through the use of additional guide curves, both the upper and lower wing are given by compounds containing more than one element, respectively. Trying to cast each of these compounds to a face yields an error.

The bug originates due to the fact that earlier, both upper and lower wings consisted of only one face, respectively. Hence, the mentioned conversion was not a problem.