[vtkusers] How to get all the edges of a tet mesh?

[Huayi Wei]

wHi, There,

I read a tet mesh by vtkUnstructuredGridReader, like following:

```python
imort vtk

reader = vtk.vtkUnstructuredGridReader()
reader.SetFileName("tet.vtk")
reader.Update()
tets = reader.GetOutput()
```

I want to get all the edges of this tet mesh. And given an edge,  I also 
need to
get all the cells which shared with this edge.

vtkUnstructuredGridGeometryFilter maybe can do such things. But I didn't 
find any
example which show such things. So I need your help, thanks very much.

Best

Huayi