中文说明：两个输入变量X，Y是n维向量，本函数通过Lagrange方法找出通过所有点的n-1阶多项式的系数，返回给P，再通过Matlab内部函数polyval计算插值
English Description：
The two input variables X and y are n-dimensional vectors. This function finds out the coefficients of n-1 order polynomials through all points by Lagrange method, returns them to P, and then calculates the interpolation by MATLAB internal function polyval