Toolpath cropping based on 3D models returns empty result

[sumpfralle]

How to reproduce:

1. generate a toolpath and select it
2. start the "crop" operation based on a 3D model
3. be notified that the result is empty (even though it should contain moves)

The problem seems to be the get_waterline_contour method in pycam.Geometry.Model.Model. It extracts all lines based on intersections between the chosen plane and all model triangles. Since these lines are not following a unified direction scheme and since their order is arbitrary, they are not properly combined into a single closed polygon, but into many open polygon fragments instead.

Maybe another library could offer a waterline algorithm for this purpose?