Infer dependence structure for samples

[annakrystalli]

Once dependence structure for sample outputs has been properly formalised according to: hubverse-org/schemas#48

We will need a utility function able to reconstruct dependence structure from data/metadata.

Input would be a model_out_tbl, and will return the “least joint” structure that can be inferred from the sample indices in that tbl.

[nickreich]

I'm not clear how this interfaces with the new proposed set-up for samples, but I'm leaving it here for further clarification.