说明：  等值线生成为了生成四边形网格的等值线,通过引入虚节点将四边形网格剖分为四个三角形实现对四边形网格上等值线的生成方法
(Isogram generate quadrilateral mesh in order to generate the contour lines, through the introduction of virtual nodes to quadrilateral mesh generation for the realization of the four triangles of the quadrilateral mesh generation method isogram)