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.