add function to compute the correlation function for q_inv
add function to compute the correlation function for q_inv

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