vtkPolyhedron: Implement GetCentroid and use it in GetParametricCenter

In VTK 9.4, the GetParametricCenter of vtkPolyhedron used to return the parametric location of its bounding box center which was always 0.5, 0.5, 0.5.

In VTK 9.5, the GetParametricCenter of vtkPolyhedron was changed to return the parametric location of the centroid of its points. This was a good change, as it allowed for a more meaningful representation of the polyhedron’s center. This was improvement over the previous implementation, but it still was not ideal especially for non-convex polyhedra.

In VTK 9.5.1, the vtkPolyhedron class introduces the GetCentroid function, which computes the signed-volume weighted centroid of a polyhedron using its pyramid decomposition. In this approach, the polyhedron is decomposed into pyramids, each formed by one of the polyhedron’s faces and a common apex point located at the centroid of all the polyhedron’s points. The GetParametricCenter method has been updated to return the parametric location of the centroid returned from vtkPolyhedron’s GetCentroid function.