说明：  pod orthogonal decomposition
( This function calculates the POD basis vectors Function inputs: X : (n x nsnap) snapshot matrix n = number of states in full space nsnap = number of snapshots cenergy : how much energy of the ensemble you want to capture, i.e. cenergy = 99.9 (percent) Function outputs: phi : (n x nsnap) matrix containing POD basis vectors lam : (nsnap x 1) vector containing POD eigenvalues Xmean : (n x 1) the mean of the ensemble X nbasis : number of POD basis vectors you should use to capture cenergy (percent) of the ensemble)