ENH: Added new data types. Restructure of composite datasets....
ENH: Added new data types. Restructure of composite datasets. vtkMultiGroupDataSet is the new superclass with two subclasses: vtkMultiBlockDataSet and vtkHierarchicalDataSet
ENH: Added new data types. Restructure of composite datasets. vtkMultiGroupDataSet is the new superclass with two subclasses: vtkMultiBlockDataSet and vtkHierarchicalDataSet