Large number of examples use interfaces that have been removed

The following files:

kruskal-telephone.cpp ;
loops_dfs.cpp ;
scc.cpp ;
reachable-loop-head.cpp ;
cc-internet.cpp ;
reachable-loop-tail.cpp ;
prim-telephone.cpp ;
dfs-parenthesis.cpp ;
edge_connectivity.cpp ;
edge-connectivity.cpp ;

All use `GraphvizGraph` and related interfaces which have been removed from the library - actually they are commented out with the note:

```
// This interface has not worked for a long time
```

So either we need to remove these examples altogether, or better, figure out what interface they should be using.

In addition strong_components.cpp calls an old Graphviz API whose meaning has changed - as a result it chokes interpreting a filename as Graphviz data. Replacing the filename with a std::ifstream just moves the error elsewhere :(