Merge ElevationProfileSegment into StreetWithElevationEdge.

Relates to #1533.

In this pull-request:
- Removal of ElevationProfileSegment class
- Integration of elevation costs factor into StreetWithElevationEdge, with default "flat" values in StreetEdge
- Externalize the ElevationProfile split, done by the client (remove one getter in StreetEdge class)

This should remove one object instance per street edge when using elevation.