vtkXMLMultiBlockDataReader: fix when reading nested multipiece datasets

When a vtkMultiBlockDataSet contains a vtkMultiPieceDataSet, vtkXMLPMultiBlockDataWriter will split each piece into another vtkMultiPieceDataSet. As a result, the written XML data file will contain nested "Piece" elements. Since vtkMultiPieceDataSets are not hierarchical, this dataset cannot be recreated as described.

To fix, have the reader replace the top-level vtkMultiPieceDataSet with a vtkMultiBlockDataSet instead when nested Piece elements are found.

Modified test to exercise writing out a vtkMultiPieceDataSet block and read it back in.

Backport: paraview/release