The authors thank the referees for helpful comments. They would also like to thank Professor Xiaowen Zhou for useful suggestions. This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11571052, 11731012), the Natural Science Foundation of Hunan Province (Grant Nos. 2018JJ2417, 2019JJ50405), the Outstanding Youth Foundation of Hunan Province Department of Education (Grant No. 18B401), the China Scholarship Council (Grant No. 201808430239), the Open Fund of Hunan Provincial Key Laboratory of Mathematical Modeling and Analysis in Engineering (Grant No. 2018MMAEZD02), and the Doctoral Scientific Research Project of Hunan University of Arts and Science.
