Funding for this research was provided by:
National Natural Science Foundation of China (National Science Foundation of China) (No. 31370843)
Text and Data Mining valid from 2018-07-24
Received: 23 May 2018
Revised: 22 June 2018
Accepted: 27 June 2018
First Online: 24 July 2018
: The authors declare no competing interests.
: This work is published under the standard license to publish agreement. After 12 months the work will become freely available and the license terms will switch to a Creative Commons Attribution 4.0 International (CC BY 4.0).