Features importance.

Ranking given according to features importance computed in the course of training. The decrease in nodes impurity, measured by the Gini index, is computed as the average among all trees.根据训练过程中计算得到的特征重要性生成排序结果。以基尼系数（Gini index）衡量的节点不纯度降低量，被计算为所有树的平均值。