[vtkusers] converting vtkImageData to vtkUnstructuredGrid

[Liam Kurmos]

I have a scalar field stored as vtkImageData.

I want to use the new vtkReebGraph object, but to build a reeb graph
we need a vtkUnstructuredGrid and a vtkDataArray.
I assume I get the vtkDataArray as the scalars used by my
vtkImageData, but how do I create a vtkUnstructuredGrid that
represents my volume?

I imagine this is straightforward to a vtk expert but is not obvious to me.

Liam