功能: 计算神经网络ISRLU激活函数

格式: netFunByISRLU(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

2. 本函数主要计算的原理如下
$$
\mathbf{netFunByISRLU}(x_i, a_i) = \begin{cases}
\dfrac{x_i}{\sqrt{1 + a_i^2x_i^2}} &x_i<0
\\
x_i &x_i\geq 0
\end{cases}
$$

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 = netFunByISRLU(x, 3.6)//回车得到如下结果
y =
[-0.25974735138355   -0.27500421631787    0.21744714338213    2.19789781453921
 -0.27729854448967    1.99957438581603    2.94959032789319   -0.27720515819341
 -0.27749067775970   -0.27051176486188   -0.27751564442354    2.98098757280083
 -0.27638896010962   -0.27731696702905   -0.27700931069011   -0.27711194734647 ]