vtkPlaneCutter to use vtkUnstructuredGridBase

For scenarios with vtkMappedUnstructuredGrid input, the vtkPlaneCutter algorithm has been changed to accept input of type vtkUnstructuredGridBase.

• Refactored out mapped unstructured grid generation into new module Testing/DataModel
• Refactored existing tests that use mapped unstructured grid, remove code duplication
• Added test for vtkPlaneCutter on mapped unstructured grid.
• Modified vtkMappedUnstructuredGrid requirements and implementation
  • void GetFaceStream(vtkIdType cellId, vtkIdList* ptIds) is now required to be implemented by an implementation used by vtkMappedUnstructuredGrid
  • added call to this for cells that require explicit face representation
• Updated vtkCPExodusIIElementBlock implemenation with GetFaceStream
  • method is there, but issues a not-implemented error.