The vtkDataArray::GetRange() method returns doubles, which have less
precision than the 64-bit integer types.  To get the range in the
native type, the following methods have been added to the subclasses
of vtkDataArray that use vtkDataArrayTemplate:
 T *GetValueRange(), void GetValueRange(T [2]), GetDataTypeValueRange(T [2])
 T GetDataTypeValueMin(), T GetDataTypeValueMax()