功能: 计算神经网络Sigmod激活函数

格式: netFunBySigmod(x)

x : 为矩阵变量或者数值

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
$$\mathbf{netFunBySigmod}(x_i) = \dfrac{1}{1 + \exp(-x_i)}$$

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 = netFunBySigmod(x)//回车得到如下结果
y =
[ 0.32455810273089    0.12445231768615    0.55414859362587    0.90006057503343
  0.00881231976048    0.88075238397570    0.95024412262233    0.01313277419531
  0.00222695825068    0.23316893829281    0.00167638705679    0.95170778045917
  0.05911024405557    0.00803028566687    0.02350794593222    0.01790377216447 ]