Refactor the access policies to reflect DUOS/TDR #26
Description
Right now the Study-Dataset-Snapshot reality of DUOS and TDR aren't clearly reflected by the model's Study-Access Policy approach. Let's realign these with the AnVIL-specific use cases in mind, making sure there's a clear representation of these entities.
Seems like the right approach would be to have Studies + "datasets", which end up looking like dataset+snapshot in a single-cloud world. The snapshots are critical for access, but the real unit of difference is likely the "dataset". Datasets are used in DUOS, but they point to "data locations", ie, the snapshots. TDR Datasets are inaccessible by users, but that's ok: they'll hold the snapshot(s) which contain real access details.