Iterate over the selected sets to list the elements

The ListDatasetElements now supports filtering by one set. If all sets were not selected, we should iterate over the selected sets and accumulate all ListDatasetElements results.

Workflow:

• Case 1 (dataset.sets == dataset.selected_sets): nothing changes, call ListDatasetElements as right now
• Case 2 (dataset.sets != dataset.selected_sets): make one call to ListDatasetElements per set in dataset.selected_sets and return the results of all these calls in one big iterator.

The call is done here.