功能: 计算神经网络PRELu激活函数

格式: netFunByPReLu(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
$$
\mathbf{netFunByPReLu}(x_i, a_i) =\begin{cases}
a_ix_i&,x_i <0
\\
x_i&,x_i\geq 0
\end{cases}
$$

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 = netFunByPReLu(x, 3.6)//回车得到如下结果
y =
[-2.63844923788609   -7.02333706003769    0.21744714338213    2.19789781453921
 -17.0019114691773    1.99957438581603    2.94959032789319   -15.5499283966375
 -21.9776011061750   -4.28581177447262   -23.0019716484481    2.98098757280083
 -9.96271823801228   -17.3393009086788   -13.4158612476736   -14.4168403289359 ]