计算法向量三角

代码说明：

This function takes as input a 2D unrestricted triangulation and computesthe normal vector of the surface.Input :          "XYZ" is the coordinate of the vertex of the triangulation (nx3 matrix).          "TRI" is the list of triangles which contain indexes of XYZ (mx3 matrix).          "strPosition" is the position where the normal is computed. It          could be "center-cells" for a computation on the center of each        &nb