def save(filename, format = 'png', layout = 'dot')
      Libgvc.gvLayout(@@gvc, ptr, layout.to_s)
      Libgvc.gvRenderFilename(@@gvc, ptr, format.to_s, filename);
      Libgvc.gvFreeLayout(@@gvc, ptr)

      nil
    end
def save(filename, format = 'png', layout = 'dot')
      Libgvc.gvLayout(@@gvc, ptr, layout.to_s)
      Libgvc.gvRenderFilename(@@gvc, ptr, format.to_s, filename);
      Libgvc.gvFreeLayout(@@gvc, ptr)

      nil
    end
Renders the graph to an images and saves the result to a file
 @param filename [String] the filename
 @param format [String] the image format to use, e.g. 'svg', 'pdf' etc.
 @param layout [String] the layout to use, e.g. 'dot' or 'neato' etc.
 @return [nil]