Fix kinetic energy computation for anti-protons and anti-neutrons in FastCaloSimSvcV2

While the kinetic energy of particles is generally given by E_{\text{kin}} = E - m, the kinetic energy for anti-protons and anti-neutrons should be calculated as E_{\text{kin}} = E - m +2m = E + m due to the annihilation to two photons of energy m. The current implementation aims to accomplish this by creating the truth particle state with a negative mass value. However, this is not correct as this also changes the energy of the particle and will not yield the desired quantity. As a fix, an offset variable is added to TFCSTruthState.h. The variable is initialized to zero and set to 2m in the case of anti-protons and anti-neutrons, which is then added to the kinetic energy. This ensures a proper implementation as well as backwards compatibility.