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Yong Xia; yxia@nwpu.edu.cn
As guest editors of this special issue, we would like to thank all the authors who have submitted their contributions to this special issue; without their interests and collaborations, this issue would not have been possible.
e would also like to thank all referees for their rigorous reviews with insightful and constructive comments that have definitely improved the quality of this issue.
The editors declare that they have no conflicts of interest.
Copyright © 2018 Yong Xia et al.