add function to compute vn{4}
add function to compute vn{4}

chunshen1987/HBTcorrelation_MCafterburner,chunshen1987/hadronic_afterburner_toolkit,chunshen1987/HBTcorrelation_MCafterburner,chunshen1987/hadronic_afterburner_toolkit,chunshen1987/hadronic_afterburner_toolkit,chunshen1987/hadronic_afterburner_toolkit,chunshen1987/HBTcorrelation_MCafterburner