Fix polyhedron IsConvex

The upper facet is not always properly identified by the algorithm when the edge is near vertical. The fix is based on the following topic: https://discourse.vtk.org/t/isconvex-of-vtkpolyhedron-may-give-wrong-result/13635/1

This is a simple fix that should be enough. To be even more robust, we could compute the normalized projection before comparing the "z" coordinate but it may be a bit too expensive.