PowhegControl: fixing reweighting issues

Timothee Theveneaux-Pelzerrequested to merge
atlas-physics/pmg/mcexperts/powheg-experts/athena:21.6-PowhegReweightingIssues into 21.6

This MR should fix some of the problems for PowhegControl seen in AGENE-2047

The (xml-)reweighting was eventually failing because the clobberlhe (and some of the rwl_) keyword was not exposed in some V2 processes. I checked all of them and added the keywords where they were missing.

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