The `MultiBlock` class has been renamed to `PartitionedDataSet`, and its API
has been refactored to refer to "partitions", rather than "blocks".
Additionally, the `AddBlocks` method has been changed to `AppendPartitions` to
more accurately reflect the operation performed. The associated
`AssignerMultiBlock` class has also been renamed to
`AssignerPartitionedDataSet`.
This change is motivated towards unifying VTK-m's data model with VTK. VTK has
started to move away from `vtkMultiBlockDataSet`, which is a hierarchical tree
of nested datasets, to `vtkPartitionedDataSet`, which is always a flat vector
of datasets used to assist geometry distribution in multi-process environments.
This simplifies traversal during processing and clarifies the intent of the
container: The component datasets are partitions for distribution, not
organizational groupings (e.g. materials).
Ref #405
Minimizing the need to have complex serialization code in vtkm/cont.
This is first step in moving DIY serialization code out of vtkm/cont. We
need to move that to filter so we can leverage policy correctly
serialize fields.
