Display improvement for 2D cell in basic mode.

Small PR which will improve the display (faster and better quality) when only need to display simple surfers (for instance to display curvature on boundary).
Now when using:
      viewer << DGtal::SetMode3D(surfel.className(), "Basic");
It will only display surfel ( signed (or not) Cell of dim 2) as a quad in a new list to allow to display front and back side.