add a new function to compute v2{2} with a rapdity gap
add a new function to compute v2{2} with a rapdity gap

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