Allows only one edge per node pair, edge type and site.
Deprecated old methods: edge_add(), edge_change(), and edge_delete(); introduced methods edge() and no_edge() to substitute them. (Is not recommendable to mix old deprecated method usage with new method usage, can lead to inconsistent data!)
New edge_get() method to retrieve only one edge (if exists). (Is only safe to use when edge creation is handled by edge() method, not by deprecated edge_add/edge_change)