I have converted Nate's b-spline generation to use my superclass
FeatureBSurf and added it to the GUI. Other half-ready operations as
filling and cut are commented out.
Missing:
- the fill type must be queried from the user. The data panel lets it
modify, but it is not user-friendly.
- b-spline surface is generated, but it covers only a fraction of the
supporting curves. It may lie on OCC error.
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