Draft: Clip polyline

Poly lines used to not be handled by vtkTableBasedClipDataSet. They are now supported, and the clipped cells are either VTK_LINE or VTK_POLYLINE. In addition, poly vertices are also now supported.

Handling polycells adds a little complexity in the filter. A poly cell can be clipped into an arbitrary number of cells. We have to keep track of on which subcell the clip started, and when it ends. This is done by adding a helper structure that keep track of where we are in the process. Cells that are not poly cells are not impacted much by this change: they have a specialized functor doing nothing. There is just one additional switch for each cell.