DM: Roads ideas features for 2.1+

In the Roads DM document, we had this list of features that were not included in 2.0, but could be interesting to add in 2.1:

• Add a roads_follow_illegally_in_junction() modifier:
  • Used to specify which roads are not legal.
  • Or this could be a parameter to roads_follow_in_junction().
• Add a roads_are_opposite(road1, road2) modifier.
  • Lets you find the opposite-direction of a road, if there is one.
  • road2 is null if no opposite.
  • For a single-lane bidirectional road (where the same lane is used for both directions), road1 and road2 will be the same.
• Have a version of create_route() where you can skip some of the intermediate routes.
  • Also, create_route_through_junction([road, junction, road]).
• Clarify way to specify lanes within routes.
  • For example create_route([r1, f1.junction_road, r2.lane7, f2.junction_road, r3.rightmost_lane() ]).
  • Note that this has several complications, for example driving-side-related issues.
• Clarify how to request "a route with at least 3 lanes".
  • Probably encapsulated in a min_lanes() modifier, which refers to a route consisting of a sequence of constant_lanes_road.
• Clarify routes for pedestrians etc.
  • Routes should have an expected_usage field.
  • Pedestrian routes also have junctions and related modifiers.
• Clarify free-space routes.
  • When specified as a path, is there a way to say "use splines" vs. "use shortest path" etc.?
• Decide on a fuller list of route elements.
• Clarify the meaning of along().
  • How to say “take the full route” vs. “just use part of the route”.
• In the future sidewalk could become a different type, instead of a type of lane