Add capability for intersection LTS

[mbonsma]

The current model assigns LTS to road and path segments only, not intersections. This can have an impact on routing; for instance, when a low-stress side street crosses a major road without a traffic signal, the routing algorithm may consider this a valid route even though crossing the major road is high stress.

Nodes should be assigned a stress level according to the following logic:

• in the absence of a stop sign or traffic signal, node stress is the highest stress of neighbouring edges
• a stop sign or traffic signal reduces an LTS 3 or 4 node to LTS 2
• a stop sign or traffic signal reduces an LTS 2 intersection to LTS 1