This is whu quickly answering these questions is a mistake, and why answering these questions is so important 
The script should be as below, but the answer it gives has me scratching my head:
[0.3333333333333333, 0.3333333333333333, 0.0]
#!/usr/bin/env python
import vtk
tet = vtk.vtkTetra()
pts = tet.GetPoints()
pts.InsertPoint(0, 0,0,0)
pts.InsertPoint(1, 1,0,0)
pts.InsertPoint(2, 0,1,0)
pts.InsertPoint(3, 0,0,1)
ptIds = tet.GetPointIds()
ptIds.InsertId(0,0)
ptIds.InsertId(1,1)
ptIds.InsertId(2,2)
ptIds.InsertId(3,3)
centroid = [0,0,0]
tet.GetCentroid(centroid)
print(centroid)