Correctly calculate elevation profile for partial street edges

[evansiroky]

This PR correctly calculates the way elevation profiles are calculated for partial street edges. This feature was authored primarily by @demory and I am merely making a PR request to isolate this feature out of the work done in #2664. This feature was initially implemented in the PartialStreetEdge class which has since been deleted in dev-1.x, so I don't even know for sure if this is actually needed anymore.

• issue: Link to or create an issue that describes the relevant feature or bug. Add GitHub keywords to this PR's description (e.g., closes #45).
• roadmap: Check the roadmap for this feature or bug. If it is not already on the roadmap, PLC will discuss as part of the review process.
• tests: Have you added relevant test coverage? Are all the tests passing on the continuous integration service (Travis CI)?
• formatting: Have you followed the suggested code style?
• documentation: If you are adding a new configuration option, have you added an explanation to the configuration documentation tables and sections?
• changelog: add a bullet point to the changelog file with description and link to the linked issue

To be completed by @opentripplanner/plc:

• reviews and approvals by 2 members, ideally from different organizations
• after merging: update the relevant card on the roadmap

[leonardehrenfried]

This has been fixed in OTP2 in #4033

Closing.