Funding for this research was provided by:
National Natural Science Foundation general projects, China (61672217)
National Natural Science Foundation general projects, China (61304208)
Foundation of the Science and technology project of Hunan Provincial Department of Education (15C1414)
Received: 4 August 2017
Accepted: 6 December 2017
First Online: 16 March 2018
: The authors declare that they have no competing interests.
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