光在散射介质中的传播，输入光子步数和吸收系数，输出光子的轨迹-A 3-D Monte Carlo Simulation of a photon propagating in scattering and absorbing media. Given an input of the number of steps (iterations) for the photon to take and the absorption coefficient of media, this function calculates a random walk and returns coordinates of final position and total final distance　origin.