Particle Gun Momentum Spectrum Header Fix
Code was directly copied from Gauss and produce an error in original state:
ParticleGun.Mom... FATAL MomentumSpectrum:: Exception throw: get():: No valid data at 'Gen/Header' StatusCode=FAILURE
ParticleGun ERROR : ParticleGun.MomentumSpectrum:: get():: No valid data at 'Gen/Header'
The header location has been changed to correct one.