说明：   [t, y]=EULER_forward_ODE(f, t0, y0, tend, Niter) 欧拉向前逼近法求解微分方程初值问题 f定义了函数f（t，y）的 t0时的初始值定义吨 Y0的定义y的初始值
( [t, y]=EULER_forward_ODE(f, t0, y0, tend, Niter) Euler forward approximation method to solve IVP ODEs f defines the function f(t,y) t0 defines initial value of t y0 defines initial value of y)