`data` block

To prepare the hadding of ROOT files (https://gitlab.cern.ch/paconnor/ProtoDarwin/-/merge_requests/34) without loosing the reproducibility (i.e. the metainformation), I propose to add a new block, namely data. Such a block should contain information specific to each data set that contributes to the target ROOT file, e.g. the preseed, the normalisation or the cross section, possibly the skim, or any relevant information (for CRAB jobs in DAS, the splitting mode, the lumi mask, the pileup normalisation).