"Twist along Curve Field" node

[portnov]

This node generates a vector field, which does a generic type of space
transformation. It is similar to what "Bend Along Curve Field" node does, but
rotation of space around the curve is controlled by user, not calculated by
some algorithm. So with this node you can generate a field that does a
combination of two transformations: 1) bending along some curve, and 2)
arbitrary twisting around that curve.

Example nodes setup:

result:

Same, but with "linear" interpolation mode:

Example setup for "Matrices" mode:

Result:

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.