add LAr improvements for MC to MC overlay from 21.0 branch

Merged Guillaume Unal requested to merge gunal/athena:master-LArMCOverlayFix into master

Propagate to master improvement for MC+MC overlay that were done in the 21.0 branch (add extra noise when overlayed event is not in the same LAr gain as pre-mixed pileup event), following detailed MC+MC overlay validation work last year.

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