Funding for this research was provided by:
Chongqing Three Gorges University Youth Fund (16QN11)
Text and Data Mining valid from 2018-05-10
Received: 26 January 2018
Accepted: 1 May 2018
First Online: 10 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.