The vtkUnstructuredGridBase class was introduced to allow the
vtkMappedUnstructuredGrid to be added for no-copy in-situ datasets.
Until now it has been somewhat hacked in as a variation of
vtkUnstructuredGrid. This patch makes the base class a first-class
citizen of Datasetland, with its own symbolic constant, algorithm-base,
etc.

* Add a VTK_UNSTRUCTURED_GRID_BASE define
* Update vtkDataObjectTypes to match
* Update vtkUnstructuredGridBase::GetDataObjectType()
* Update a handful of filters that accept this class as input
* Add a vtkUnstructuredGridBaseAlgorithm interface

Change-Id: Ib5d3256d6d6c3f281dc123891b16b483e26e29dc