Include token in subgraph even if not covered by segmentation node

[matthias-stemmler]

This fixes a regression I introduced in #289: When subgraph is called with a token and a segmentation, and the token is not covered by any segmentation node, then the token is not included in the subgraph.

In #289 I had filtered out matched tokens to ensure that tokens are sorted, claiming that new_overlapped_nodes_iterator already included the matched tokens. However, if a segmentation is specified, this is only true for tokens that are covered by a segmentation node.

The fix changes get_left_right_token_with_offset_with_segmentation so that instead of failing, it returns just the covered tokens without context in case none of them is covered by a segmentation node.

I hope this doesn't break any other invariants. At least the existing tests are still green. 🙂

[matthias-stemmler]

I added another test to close the (pre-existing) gap in test coverage. The remaining CI failure seems to be a permissions issue.