Fix OnDiskDataset access after index_select()

[Karl-XZ]

Fix OnDiskDataset.get() and related subset access after index_select().

Subsets created from OnDiskDataset.index_select() returned inconsistent
results between subset[idx] and subset.get(idx), because get() did not
resolve indices through self.indices().

This PR aligns get(), multi_get(), and __getitem__() for subset access
and adds a regression test.

Testing

python -m pytest test/data/test_on_disk_dataset.py -q