Fix bug on trapezoid constructor using corner points
- Symptom: GetPointOnSurface() was incorrectly returning (0,0,0)
- test/unit_tests/TestTrap.cpp is modified here to show the bug
- bug traced to incomplete initialization of TrapezoidStruct::sideAreas[4:5]
- fix was to reorder calls in UnplacedTrapezoid constructor, so that the trapezoid parameters are initialized before calling MakePlanes(corners), which also initializes sideAreas[]