Contains data defining a non-uniform rational b-spline (NURBS)
curve. All data items are used in both input and output.
There are 4 values for each control point, or pole: wx (poles[i][0]), wy
(poles[i][1]), wz (poles[i][2]) and w (poles[i][3]) (0<= i < num_poles)
Note that the first 3 coordinates are weighted.
The number of knots num_knots and poles num_poles and the order order,
are related as follows: num_knots = num_poles+order.

The _param values specify how much of the spline is active, i.e.,
visible and used. They cannot be outside the parameter range defined
by the knot vector. The lowest allowed value is ps = knots[order-1],
where [i] denotes the C-language (zero-based) subscript. The
largest is pe = knots[num_poles].

If is_rational is nonzero, the weights are not all equal to 1.0. To
create a spline, this value is ignored and the rational status is
determined from the data.

For more information on the conditions spline data must meet, see
UF_CURVE_create_spline.