Modernize copy prevention in utility/graph EdgeList/Node/Edge to use = delete (#672)
Replace old-style private-and-undefined copy constructor/assignment with
explicit `= delete` in six internal classes across `Graph.hh` and
`Digraph.hh`:
- `Graph.hh`: `EdgeList`, `Node`, `Edge`
- `Digraph.hh`: `DirectedEdgeList`, `DirectedNode`, `DirectedEdge`
Also corrects the copy assignment signatures from the non-standard `T&`
argument to the correct `T const &` form. The `Graph` class itself
(which has a real, implemented copy constructor and assignment) is
unchanged.