目标检测MAP，Recall，Precision

对于MAP较好的解释：
https://datascience.stackexchange.com/questions/25119/how-to-calculate-map-for-detection-task-for-the-pascal-voc-challenge

对于Recall和Precision较好的解释：
https://blog.csdn.net/liuweiyuxiang/article/details/80111226
https://www.zybuluo.com/Team/note/1175689