Funding for this research was provided by:
Fundamental Research Funds for the Central Universities (31920180116, 31920180044, 31920170072)
National Natural Science Foundation of China (31260098, 31560127)
the Program for Yong Talent of State Ethnic Affairs Commission of China (No.[2014]121)
Gansu Provincial first-class discipline program of Northwest Minzu University
Article History
Received: 12 June 2018
Accepted: 21 October 2018
First Online: 26 October 2018
Competing interests
: The authors declare that they have no competing interests.