Because vtkConnectivityFilter may produce a vtkPolyData as output,
change the base class from vtkUnstructuredGridAlgorithm to
vtkPointSetAlgorithm. This provides convenience methods for accessing
the output as different types, e.g., GetUnstructuredGridOutput() and
GetPolyDataOutput().

This will break backwards compatibility where callers to
vtkConnectivityFilter::GetOutput() assign the value returned to a
vtkUnstructuredGrid pointer or invoke uninherited API in
vtkUnstruturedGrid from a temporary returned by GetOutput(). Changing
the call from GetOutput() to GetUnstructuredGridOutput() is that path
forward in such cases. Note that GetUnstructuredGridOutput() will
return nullptr if the input to vtkConnectivityFilter is a vtkPolyData
(its output will be a vtkPolyData).