VTK/Composite data changes

Primarily motivated by better support of AMR data structures, various APIs related to vtkCompositeDataSet have been changed.

Refactoring

vtkCompositeDataSet has been primarily used a container of data sets, even though it really represents a tree of data sets. So we first renamed the class vtkDataObjectTree, then exctracted the methods that support simple container functionalities and put them into a fully abstract class vtkCompositeDataSet. The refactored hierarchy is shown below. It should be noted here that vtkUniformGridAMR now provides a new implementation. The details can be found in a later section.