Fixing MG5_aMC NLO Gridpack Systematics

Hannes Mildnerrequested to merge
hmildner/athena:21.6-mg-gridpack-nlosyst into 21.6

For NLO gridpacks, the systematics module is no longer launched by generate_events but run standalone after the events have been generated. According to the authors it's "saver" this way and indeed it avoids a crash we've been observing.

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