The authors would like to thank Professors Dachun Yang and Wen Yuan for sending us some of their papers. Qixiang Yang is supported by NSFC, 11571261 and Hua Wang is supported by NSFC, 11771165 and 11571131.
National Natural Science Foundation of China, Grant/Award Number: 11101172, 11371158, 11571131 and 11571261