vtkCompositeDataPipeline does not support CompositeOutput when iterating over input
vtkCompositeDataPipeline is a very useful executive that is able to handle composite input for any filter that is not composite-aware, as long as the filter takes a vtkDataSet as input and outputs a vtkDataSet.
However, if the filter output a vtkPartitionedDataSetCollection, then compositeOutputs[port]->SetDataSet(iter, outObj);
While this works with vtkMultiBlockDataSet, for PartitionedDataSetCollection, the result will be an intricated vtkDataAsssembly, which we definitely do not want.
the vtkCompositeDataPipeline should detect that case and handled it differently, merging the composite datasets together and using vtkDataAssembly::AddSubtree
This looks like the correct approach as the current approach is inintuitive at best and problably bug-prone and would make must filter crash after.