Structure definition for curvature on curve data. It includes all the input
elements to create a smart offset object with the curvature on curve method.

The curvature will be evaluated at a curve parameter. A scalar object needs
to be created for the parameter.

Note an offset object contains a vector of three components. For the
curvature on curve subobject, the curvature vector is identified by the
vector of the object. That is, the curvature value is represented by the
magnitude of the vector and the curvature direction (the normal direction
of the curve at that parameter) is represented by the direction of the
vector.