CG:CG_elem.cpp: Fix in the triangulation of a polygon

[kgkara]

So far, this took place by creating triangles with the 1st vertex of the polygon and two other vertices of the polygon. In case these 3 vertices are collinear, the debug mode does not complete its execution. Now, the approximate barycenter of the polygon is computed and is used along with two vertices of the polygon to create the triangles. Closes #355