(user): Add a section on costs (vehicle arc costs, span costs, disjunctions costs).
(user): rename Start to Synchronize ?
(user): decouple the iterating from the defining of a neighbor.
(user): Add option to prune neighbords where the order of node pairs is violated (ie precedence between pickup and delivery nodes).
(user): Move this to local_search.cc if it's generic enough.
(user): Detect pairs automatically by parsing the constraint model; we could then get rid of the pair API in the RoutingModel class.