vtkMultiBlockDataSet and parallel pipelines

[utkarshayachit]

@scotsman60

I’m already using vtkCompositePolyDataMapper2 - and as long as all of the blocks are PolyData this works, but if any are UnstructuredGrids it just skips them.

For that, use the standard GeometryFilter or DataSetSurfaceFilter to convert UGs to PD. Those filters do each block separately and don’t bother appending them together – which saves time.