finding index via iteration

[yakra]

• Can be deleted now; apparently was unused even in the first commit
  • [x] e7ff789 HighwaySegment::index()
• Not too big a deal; only used when datacheck errors are flagged
  • [ ] HighwaySystem::route_index
  • [ ] HighwaySystem::con_route_index

If changes are made to store arrays of objects rather than pointers, indices can be found via pointer arithmetic.