vtkArrayDownCast will downcast a vtkAbstractArray to a more
derived class using FastDownCast, if available. If FastDownCast
is not supported by the the array, the slower SafeDownCast is
used instead.

The vtkArrayDownCast_FastCastMacro and
vtkArrayDownCast_TemplateFastCastMacro are used to tell
vtkArrayDownCast when FastDownCast is available for a given array
(or set of templated arrays).