Wrong computation of first (centroid) moment of Polyhedron

**For the following Polyhedron, the `centroid` is at `(1.5, 1.5, 1.5)` but instead `PolyClipper` gives `(1.500000, 1.125000, 1.125000)`:**

```
import PolyClipper as pc

dummy_vertex_list = [[0.0, 0.0, 0.0], [3.0, 0.0, 0.0], [0.0, 3.0, 0.0], [3.0, 3.0, 0.0], [0.0, 0.0, 3.0], [3.0, 0.0, 3.0], [0.0, 3.0, 3.0], [3.0, 3.0, 3.0]]
dummy_all_vertex_neighbor_list = [[1, 2, 4], [0, 3, 5], [0, 3, 6], [1, 2, 7], [0, 5, 6], [1, 4, 7], [2, 4, 7], [3, 5, 6]]

polyhedron_vertices = [pc.Vector3d(*coords) for coords in dummy_vertex_list]
poly = pc.Polyhedron()
pc.initializePolyhedron(poly, polyhedron_vertices, dummy_all_vertex_neighbor_list)
print("poly:", poly)
print("moments: ", pc.moments(poly))
```

The output obtained is:
```
('poly:', Polyhedron[Vertex3d[ ( 0 0 0) [1 2 4 ] 1 -1], Vertex3d[ ( 3 0 0) [0 3 5 ] 1 -1], Vertex3d[ ( 0 3 0) [0 3 6 ] 1 -1], Vertex3d[ ( 3 3 0) [1 2 7 ] 1 -1], Vertex3d[ ( 0 0 3) [0 5 6 ] 1 -1], Vertex3d[ ( 3 0 3) [1 4 7 ] 1 -1], Vertex3d[ ( 0 3 3) [2 4 7 ] 1 -1], Vertex3d[ ( 3 3 3) [3 5 6 ] 1 -1]])
('moments: ', (9.0, (1.500000, 1.125000, 1.125000)))
```

**Where am I going wrong? Please help!!**

Start by running the supplied Python reproduction, focusing on initializePolyhedron and pc.moments for the cube vertices and neighbor lists. Compare the returned volume and centroid with the expected cube values; done means the reported centroid is (1.5, 1.5, 1.5) for this case.

Written by the indexing model from the issue text.