re-write completely extract_skeleton() function

    change the way we think: for each unitary rectilinear edge,
        check if it is an convex or concave edge
        and add only thoses
        TODO: very fiew particular edges are detected as convex but they
        aren't
    use real step to retrieve points, not div 2 (it was to show inside)
    don't show points, just the relevent edges of the structure
fix: some convex edges weren't detected

    use monstruous ifs for readability's sake
fix indentation
add direct projection of edges function
show only the newly projected edges

    not thoses who were used to generate the 3D structure
change color of the structure's edges to white