Record photon energy variation in the metainfo

Most variations available in a tree are recorded in the metainfo:

variations
{
    event_rec
    {
         ScaleFactorTight_UL2018_stat_up
         ...
    }
    MuonRecWgts
    {
         NUM_TightID_DEN_TrackerMuons_abseta_pt_stat_up
         ...
    }
    MuonScales
    {
         statup
         ...
    }
    PhotonRecWgts
    {
         ID_statData_up
         ...
    }
}

This structure could be used to generalize GetVariations, but unfortunately photon scale variations are missing from the list because they are already stored in the Ntupliser. I propose to harmonize this by storing the relevant metainfo already in the Ntupliser. This requires moving metainfo creation one step up in the chain so it's a significant change, although it should be mostly transparent (old code will work on new ntuples and new code only needs the old mergeNtuples).

Since we have existing ntuples without this information, it would also be nice to have an adapter to add the info to the trees.