tokens: We thank the anonymous referee for his/her constructive comments. This work is supported by the National Basic Research Program (“973” Program) of China under grant no. 2014CB845800 and the National Natural Science Foundation of China (grant no. U1331202, 11533033, U1331101, 11573014, 11422325, and 11373022). D.X. acknowledges support from the One-Hundred-Talent Program of the National Astronomical Observatories, Chinese Academy of Sciences.
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