说明：  fem2d_project读取定义采样一个文件（标量或矢量）函数的3个参数，以及一个节点和四面体元素的列表用于数据的有限元表示的 计算与之相关联的一组有限元系数给定的有限元素网格，并将该信息写入文件。 使有限元表示由节点、元素和值构成的文件。
(FEM2D_PROJECT reads files defining a sampling of a (scalar or vector) function of 3 arguments, and a list of nodes and tetrahedral elements to use for a finite element representation of the data. It computes a set of finite element coefficients to be associated with the given finite element mesh, and writes that information to a file so that an FEM representation is formed by the node, element and value files.)