2017-9-5 by zhou xiang.
Including four fixed-step numerical methods: Forward_Euler, Backward_Euler, Trapezoid, Improved_Euler, RK_method. two adaptive time step methods: Hybrid_method and CCL.
Reference: Chen M H, Chen P Y, Luo C H. Quadratic adaptive algorithm for solving cardiac action potential models[J]. Computers in Biology & Medicine, 2016, 77:261-273.