功能: 计算神经网络SQNL激活函数

格式: NetFunBySQNL(x, a)

x : 为矩阵变量或者数值
a : 为数值或者1*1的矩阵变量

说明:
1. 函数执行成功返回和x同纬度的矩阵变量, 返回值的范围在[-1, 1]

2. 程序内部计算时,a的取值为其绝对值

3. 本函数主要计算的原理如下
$$
\mathbf{NetFunBySQNL}(x_i, a_i)=\begin{cases}
1 &,x_i > a_i
\\
\dfrac{2x_i}{a_i} - \dfrac{x_i^2}{a_i^2} &,x_i >0
\\
\dfrac{2x_i}{a_i} + \dfrac{x_i^2}{a_i^2} &,x_i \geq -a_i
\\
-1&,x_i<-a_i
\end{cases}
$$
例子:
x =
[-0.73290256607947   -1.95092696112158    0.21744714338213    2.19789781453921
 -4.72275318588258    1.99957438581603    2.94959032789319   -4.31942455462153
 -6.10488919615973   -1.19050327068684   -6.38943656901337    2.98098757280083
 -2.76742173278119   -4.81647247463300   -3.72662812435377   -4.00467786914887 ]
y = NetFunBySQNL(x, 3.6)//回车得到如下结果
y =
[-0.36572162842641   -0.79016652102193    0.11715556884153    0.84831091524121
 -1.00000000000000    0.80236403190307    0.96735858475539   -1.00000000000000
 -1.00000000000000   -0.55203128946212   -1.00000000000000    0.97043392090841
 -0.94651338186380   -1.00000000000000   -1.00000000000000   -1.00000000000000 ]