说明：  给定控制点，绘制NURBS曲面，并计算其恰好不发生自交的等距曲面,均匀变化的等距曲面和产生自交的等距曲面，绘制高斯曲率、平均曲率、主曲率等。
(Given control points, draw a NURBS surface, and calculate the equidistant surface without self-intersection, the equidistant surface with uniform change and the equidistant surface with self-intersection, and draw Gaussian curvature, average curvature, principal curvature, etc.)