Computes a contour object from the given Woolz object and saves it
      using the VTK ascii polydata format.
      The input object is read from stdin and output data are written
      to stdout unless filenames are given.
      Example: WlzExtFFContourObj -o out.vtk -i -v 100.0 in.wlz
      The input Woolz object is read from in.wlz, and the iso-value
      (iso-value = 100.0) contour is written in VTK ascii polydata format
      to out.vtk.