Funding for this research was provided by:
National Natural Science Foundation of China (61701037)
Beijing Municipal S&T project (Z181100003218003)
Beijing Natural Science Foundation (L172033)
111 Project of China (B16006)
Article History
Received: 31 December 2016
Accepted: 7 February 2019
First Online: 26 February 2019
Competing interests
: The authors declare that they have no competing interests.
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