Funding for this research was provided by:
National Natural Science Foundation of China (81371006)
Received: 8 January 2019
Accepted: 8 July 2019
First Online: 16 July 2019
Ethics approval and consent to participate
: Not applicable.
: All the authors have read and approved the paper and declare no potential conflicts of interest in the paper. If their paper is accepted, all the authors will observe the terms of the publishing license.
: The authors declare that they have no competing interests.