Surface helper tuto fill contours

Add doc on filling contour region in complement of Surface class from topology/helper.
A new example in link to the freeman2pgm tool of DGTalTools and new function uComputeExterior.
The test was also updated and the contour tests/samples/contourS.fc was updated (since he was obtained with an inverted cell selection).

SCellSet -> insist in the fact that it should reflect a closed surface.