`is_connected()` is too slow

is_connected function is documented as

```
// Is the undirected graph connected?
// Is the directed graph strongly connected?
```

and achieves this goal by checking if each vertex is reachable from each other one. This is unnecessarily resource-consuming and slow, O(n^3) (or even O(n^4)). You could call this a performance bug.

Please see PR397 (https://github.com/boostorg/graph/pull/397) that fixes this to run in O(N) (O(N+E)).

See the attached log_vec.txt for speed evaluation.
[log-vec.txt](https://github.com/user-attachments/files/17717572/log-vec.txt)
(please tell if you're interested in the program that generates it)