Add setting for configuring output of PGUN simulation (@adavis)

Merged Marco Cattaneo requested to merge cherry-pick-076be4a8-6 into digi14-patches

See merge request !2393 (merged) (cherry picked from commit 076be4a8)

Needed to propagate pGun PV from Gauss through the Digitisation processing step for Sim09 pGun productions with Gauss v49r16 or later. To be activated for steps that process pGun, by setting SimConf().pGun=True

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