S4 container for a multi-study QTL analysis: a collection of
individual-level QtlDataset studies, optionally combined with a
QtlSumStats collection of summary-statistic-only studies. Used as
the input to multi-study fine-mapping and colocboost-style analyses.
At least two studies must be present in total, counting qtlDatasets
entries plus the studies in sumStats.
For traits that appear in more than one qtlDatasets entry, the
per-trait genomic positions must agree across the entries (enforced by
validity). No cross-checking is performed against sumStats variant
positions, since summary statistics are already computed and cannot be
re-aligned at construction time.