Funding for this research was provided by:
National Natural Science Foundation of China (61571073)
Chongqing Municipal Education Commission (Kjzh11206)
Text and Data Mining valid from 2018-05-16
Received: 12 February 2018
Accepted: 30 April 2018
First Online: 16 May 2018
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.