CenterOfMass is not working in tvtk

in tvtk the center of mass allways returns (0,0,0)
**Code tvtk:**
```
from tvtk.api import tvtk

points = tvtk.Points()

top = 1
bottom = .1
points.insert_next_point(0,0,1)
points.insert_next_point(top,0,1)
points.insert_next_point(top,top,1)
points.insert_next_point(0,top,1)
points.insert_next_point(0,0,0)
points.insert_next_point(bottom,0,0)
points.insert_next_point(bottom,bottom,0)
points.insert_next_point(0,bottom,0)


polydata = tvtk.PolyData()
polydata.points = points

cofm = tvtk.CenterOfMass()
cofm.set_input_data(polydata)
cofm.use_scalars_as_weights = False
cofm.update()
print(cofm.center)
```
**Code vtk works:**

```import vtk
points = vtk.vtkPoints()
top = 1
bottom = .1
points.InsertNextPoint(0,0,1)
points.InsertNextPoint(top,0,1)
points.InsertNextPoint(top,top,1)
points.InsertNextPoint(0,top,1)
points.InsertNextPoint(0,0,0)
points.InsertNextPoint(bottom,0,0)
points.InsertNextPoint(bottom,bottom,0)
points.InsertNextPoint(0,bottom,0)


polydata = vtk.vtkPolyData()
polydata.SetPoints(points)

cofm = vtk.vtkCenterOfMass()
cofm.SetInputData(polydata)
cofm.SetUseScalarsAsWeights(False)
cofm.Update();
cofm.GetCenter()

Start by reproducing the example with tvtk.Points, tvtk.PolyData, and tvtk.CenterOfMass, then compare its behavior with the corresponding vtk.vtkCenterOfMass calls shown in the issue. Trace the tvtk wrapper path for set_input_data, use_scalars_as_weights, update, and center; done means the example returns the expected nonzero center like the direct VTK version.

Written by the indexing model from the issue text.