Suppress OnRoute transfers

Splitting out from https://github.com/mapbox/directions-transit/issues/66:

When a trip contains an OnRoute transfer, we currently would issue a new leg for this trip, since the line changes. In reality, we stay on the same line, though. We should suppress these types of transfers in our output, as it exposes internals right now, which should be hidden from the user.

Tasks:

- [ ] add a test fixture that contains an actual loop in the middle
- [ ] implement suppression of on-route transfers

Start by inspecting the transfer-output code and existing test fixtures in the nepomuk repository, then find how OnRoute transfers are represented for a trip. Add a fixture containing an actual loop in the middle and verify that OnRoute transfers are suppressed while ordinary line-changing transfers remain visible.

Written by the indexing model from the issue text.