"Prepare NURBS Curves Net" node

[portnov]

Addressed problem description

If you want to build a Gordon surface (by use of "Surface from NURBS Curves Net" node), it is required that your curves intersect exactly. If your curves are not generated, but manually created (and introduced into Sverchok tree by use of "Bezier In" or "NURBS Input" nodes), it is hard to adjust curves manually so that they would exactly intersect.

Solution description

With this new node, you may say that, for example, U curves are "primary", and the surface should follow them exactly, and V curves are "secondary", they provide only approximate shape. Then the node will slightly adjust your V curves so that they would exactly intersect your U curves. Or vice versa, you can say that V curves are primary.

Initial curves net (note that curves do not intersect exactly):

Here we say that U curves are primary. Black curves are original, red and blue curves are result of adjustment by the node. Note that now the curves intersect exactly.

And here we say that V curves are primary:

Simple example with "Crop" disabled:

And the same with "Crop" enabled:

Preflight checklist

Put an x letter in each brackets when you're done this item:

• Code changes complete.
• Code documentation complete.
• Documentation for users complete (or not required, if user never sees these changes).
• Manual testing done.
• Unit-tests implemented.
• Ready for merge.