`vtkHyperTree` global index refactoring

There are 2 ways to index data in a vtkHyperTree.

• Explicit indexing: activated by calling SetGlobalIndexFromLocal
• Implicit indexing: activated by calling SetGlobalIndexStart

If one calls one of those 2 methods, then the getter of the other indexing method crashes because of an assert. The design is flawed: you should be able to instantiate one version or another, and not be able to change its behavior at run-time. The solution to this is to do that by inheritance, having for example a vtkExplicitHyperTree and a vtkImplicitHyperTree implementations inheriting from abstract class vtkHyperTree.