COMP: Fix "was hidden" warning -Woverloaded-virtual

Update() function in derived class was hiding methods from base class: Update(foo), Update(foo, bar), etc.

This is because the way cpp lookup works, more info: https://stackoverflow.com/questions/6727087/c-virtual-function-being-hidden

vtkAlgorithm.h warning: ‘virtual void vtkAlgorithm::Update(int)’ was hidden [-Woverloaded-virtual]

vtkAlgorithm.h:569:15: warning: ‘virtual int vtkAlgorithm::Update(vtkInformation*)’ was hidden [-Woverloaded-virtual]
   virtual int Update(vtkInformation* requests);
               ^~~~~~
vtkITKImageThresholdCalculator.h:80:8: warning:   by ‘virtual void vtkITKImageThresholdCalculator::Update()’ [-Woverloaded-virtual]
   void Update() VTK_OVERRIDE;
        ^~~~~~