Metadata to indicate the "file" for matching between external TTree (e.g., WeightTree)

[jedori0228]

During the "ND CAFMaker / CAFAnalysis Stakeholder Scrum meeting", to properly match between CAF and per-neutrino-entry-Tree (e.g., a separate TTree with cross-section reweights saved), we need an info that indicates the "file", in addition to SRTrueInteraction::genieIdx.