change range() to xrange()
move _extract_skeleton() in it's caller

    make stuffs a little bit more readable
change a _get_edges() call to be more consistant
add projections as Interval2D on Interval3Ds
use Interval2Ds for projection

    remove get_projected_segments() which were really crappy
change projection removal: revert if and for
add test to detect non-connected structures

    when not connected :
        print warning
        Interval3D and it's projections are empty