Fix missing line segment on non-manifold meshes

[julienr]

This is a work in progress to fix the issue in #20.

Todo:

• [x] Test changes on other meshes
• [x] Add regression test

[julienr]

With these changes, we get a single polyline, which looks good

[julienr]

On the sphere, there is still the missing segment, which is a different issue (closing the line by making last point being first point). I feel for this, the best may be to have an option 'close_line'. Or just a helper function.