The authors are most grateful to the Editor and the anonymous referees for carefully reading the paper and for offering valuable suggestions, which greatly improved this paper. This paper is supported by the National Nature Science Foundation of China (71963008), the Science and Technology Plan Project of Hunan Province (2016TP1020), Application-Oriented Characterized Disciplines, Double First-Class University Project of Hunan Province (Xiangjiaotong [2018]469), the Scientific Research Fund of Hunan Provincial Education Department (18C0660),
the National Statistical Science Research Project of China (2018LY05), the Guangxi Provincial Natural Science Foundation of China (2018GXNSFAA294131) and the Discovery Grants (RGPIN-2018-06466) from Natural Sciences and Engineering Research Council of Canada.
