Funding for this research was provided by:
the Scientific Research Foundation of Hunan Provincial Education Department (16A198)
the Hunan Provincial Natural Science Foundation of China (2015JJ6101)
NSFC (11571207)
Article History
Received: 2 May 2018
Accepted: 6 August 2018
First Online: 16 August 2018
Competing interests
: The authors declare that they have no competing interests.