int tet_faceindex(int faceno, int vtxno)
-1 if an invalid number is specified.
3 to refer to the four vertices of a generic tetrahedron.
The face on the tetrahedron. Face 0 is the triangle that doesn’t have vertex 0.
Which vertex on the triangle to return,
2. Starts with
the lowest number and follows Houdini’s winding convention, ie,
face 0 is vertices 1, 2, and 3.