Nodes are the vertices where three or more arcs meet. An arc can exist without including
any nodes, a path that has no neighbouring relationship with another path.
This is not the same terminology as used by other systems, such as "arc-node". The
arc_link_vertex
mapping is inherently ordered, but we don't consider order of arcs.
Duplicated arcs (i.e. complementary turns around neighbouring polygons) are not kept.
The object_link_arc
mapping records which arc belongs to the objects, so feature polygons
can in theory be reconstructed within objects by tracing arc_link_vertex
start and end point
identity.