[vtkusers] convert a multi-block structured grid into a single unstructured grid

[Huayi Wei]

Hi, There,

I have a multi-block grid and I want to convert it into a single 
unstructured grid.

In the multi-block grid, the block size maybe different.  I am trying to 
use vtkAppendFilter
to do such thing.

```python
reader = vtk.vtkTecplotReader()
reader.SetFileName("mesh.dat")
reader.Update()

mesh = reader.GetOutput()

apd = vtk.vtkAppendFilter()
apd.MergePointsOn()

N = mesh.GetNumberOfBlocks()
for i in range(0, N):
       apd.AddInputData(mesh.GetBlock(i))

apd.Update()
umesh = apd.GetOutput()

print umesh.GetNumberOfPoints()
print umesh.GetNumberOfCells()

```

For the first and second blocks, their sizes are same (30*5) , 
vtkAppendFilter can work well. But the third block size is 15*4 and the 
fourth block size is 8*15. Then I get the following error message:

```
ERROR: In 
/home/why/repositories/vtk/VTK/Common/ExecutionModel/vtkStreamingDemandDrivenPipeline.cxx, 
line 857
vtkStreamingDemandDrivenPipeline (0x1f5e4c0): The update extent 
specified in the information for output port 0 on algorithm 
vtkTrivialProducer(0x1f5e090) is 0 30 0 5 0 0, which is outside the 
whole extent 0 15 0 4 0 0.

ERROR: In 
/home/why/repositories/vtk/VTK/Common/ExecutionModel/vtkStreamingDemandDrivenPipeline.cxx, 
line 857
vtkStreamingDemandDrivenPipeline (0x1f5f070): The update extent 
specified in the information for output port 0 on algorithm 
vtkTrivialProducer(0x1f5ec40) is 0 30 0 5 0 0, which is outside the 
whole extent 0 8 0 15 0 0.
```

So what is the mistake I made? I need your help and suggestions, thanks 
very much.

Best

Huayi