说明：  fit_mix_2D_gaussian - fit parameters for a 2D mixed-gaussian distribution using EM algorithm format: [u,covar,t,iter] = fit_mix_2D_gaussian( X,M ) input: X - input samples, Nx2 vector M - number of gaussians which are assumed to compose the distribution output: u - fitted mean for each gaussian (each mean is a 2x1 vector) covar - fitted covariance for each gaussian. this is a 2x2xM matrix. t - probability of each gaussian in the complete distribution iter - number of iterations done by the function