功能: 计算神经网络ELU激活函数

格式: NetFunByELU(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
$$
\mathbf{NetFunByELU}(x_i, a_i) = \begin{cases}
a_i(\exp(x_i) - 1) &,x_i<0
\\
x_i&,x_i\geq 0
\end{cases}
$$ 

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 = NetFunByELU(x, 3.6)//回车得到如下结果
y =
[-1.87015591636346   -3.08828789942525    0.21744714338213    2.19789781453921
 -3.56799359821535    1.99957438581603    2.94959032789319   -3.55209285923485
 -3.59196505681452   -2.50535449100695   -3.59395487262226    2.98098757280083
 -3.37383441869185   -3.57085694453868   -3.51333405632592   -3.53437142108349 ]