ExtractIsocurve through Greville points

Although not very impressive on its own, the output of spb_ExtractIsocurvesThruGrevillePts.py can be used by _Sweep1 to create the input surface. Before doing so, the shape curves can also be limitedly modified to adjust the shape of the surface without changing its knot structure. Specifically, the knot structures of all curves need to be maintained, and the shape curves must continue to intersect the rail curve at the latter’s Greville points.

Some _MatchSrf solutions may also be possible by using these “Greville curves”.