tokens: We would like to thank the anonymous referee for instructive comments. This work is partially supported by the National Natural Science Foundation of China (NSFC 11433004, 11103016, 11173020), the Doctoral Fund of the Ministry of Education of China (RFDP 20115301110005), the Key Project of the Chinese Ministry of Education (212160), the Top Talents Programme of Yunnan Province, the Applied Basic Research Project of Yunnan Province (2011FB012) and the Light of the Western Talent Training Project (W8090303).
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