[vtkusers] Problem with surface reconstruction and contour filter in C++.

[John Biddiscombe]

> 'vtkDataSetToPolyDataFilter::SetInput' : cannot convert parameter 1 from
> 'vtkImageData *' to 'vtkDataSet *'
>
>         Types pointed to are unrelated; conversion requires
> reinterpret_cast, C-style cast or function-style cast

Recent cleanups of all the vtk classes have removed #include "stuff.h" from
many headers and you obviously haven't got a
#include "vtkImageData.h"
statement at the top of your file. Hence the compiler doesn't know it's a
subclass of vtkDataSet. All the headers just state
class vtkImageData;
now so you need to add it. It's a pain, but it make the build process
cleaner and faster (and it;s the right thing to do really)

JB