Implementation of antiproton generator in Geant4

A generator of antiproton ($\bar{p}$) in Geant4 has been developed using a theoretical model with the consideration of near-threshold treatment and used preliminarily in COMET experiment. The precision of this generator has been verified by comparing with experimental results taken in CERN (1970) and ITEP (1994). By implementing this generator into official data analysis framework of COMET, the yield of $\bar{p}$ in COMET experiment can be evaluated properly. Background caused by $\bar{p}$ can also be estimated by this generator in future, which will instruct the design and optimization. Moreover, this generator can also be used in the simulation of other experiments involving near-threshold $\bar{p}$ generation.