Fix the collapse procedure

[GoogleCodeExporter]

There is a bug that sometimes prevents the surface_to_skeleton filter to 
convert the
degenerate manifold into a network of curves.

It would be simpler to remove the skeleton_model and instead use a temp model
for the execution of the filter (no wing-edge required)

Original issue reported on code.google.com by andrea.tagliasacchi on 3 Feb 2013 at 3:38