change: all classes extends at least object
add concaveness to edges

    add class Edge3D derived from Segment3D, add type of edge
    _edge_eligible() now returns 0, 1 or 2 depending on the type
    _extend_adjacent_edges() only extend edges with the same type
refactor eligible test and abstract ourselves from edge's types value
use Edge3Ds instead of tuples when treating w/ edges
add: color concave edges in yellow, rest in white

    also remove random comment that were commited by mistake
    in commit f07f6b6