Add setting for configuring output of PGUN simulation (@adavis)
See merge request !2385 (merged) (cherry picked from commit a95d551b)
Needed to propagate pGun PV from Gauss through the Brunel processing step for Run 1 pGun productions with Gauss v54r0 (Sim10) and Gauss v49r16 (Sim09) or later. To be activated for steps that process pGun, by setting SimConf().pGun=True