Computes (x,y,z) components of triangle centers (centroids).
The (x,y,z) coordinates of vertices are packed into the specified
array xyz. The number of vertices is nv = xyz.length/3.
Triangles are specified by triplets of vertex indices (i,j,k), one
triplet per triangle, packed into the specified array of integers
ijk. The number of triangles is nt = ijk.length/3.
The returned array of packed (x,y,z) coordinates of triangle centers
has length equal to ijk.length. Each center (centroid) is the average
of the corresponding triangle vertices.