Funding for this research was provided by:
NSF of China (11301137)
NSF of Hebei Province, China (A2014205100)
Educational Commission of Hebei Province, China (ZD2014062)
Article History
Received: 21 September 2016
Revised: 17 April 2017
Accepted: 18 April 2017
First Online: 8 May 2017