Notes: MatConvNet II vl_simplenn

this article serves as the second in the series of Notes on MatConvNet.

1. Notes on MatConvNet(I) – Overview
   Here I will mainly introduce the central of the matcovnet—vl_simplenn.
   This function plays a quite important role in forward-propagation and backward-propagation.
   PS. I have to admit that writing blogs in Chinese would bring much more traffic than writing English blogs. Yet, if I care too much about vain trifles, it is a sad thing.

Something that should be known before

仅涉及BP算法的基本原理介绍。为了更好地理解后续内容，请仔细阅读BP Algorithm这篇博客。这也是BP算法的经典入门材料之一。如果您已经完成了这篇博客的阅读，请继续关注本系列的第二篇。为了