Implement connected_subgraphs()

[merkys]

This PR introduces connected_subgraphs() which generates all the connected subgraphs of a given graph. I needed this for my applications, and I was unable to find anything similar even in Python's NetworkX.

Apparently, there does not seem to be a trivial algorithm for that. Intuitive solution of checking each of 2^N vertex subsets for connectivity is quite expensive and inefficient for sparse graphs.

My implementation is based on a modified BFS starting at each vertex and forking off on every neighbour. Intuitively (= no proof) this should work on directed and multiedge graphs, as neighbours() is used in this modified BFS.

Current implementation could be made more efficient by using something else than the difference of Set::Object objects to find the next expansion tier for the modified BFS. Maybe a matrix form. But for small graphs this does the trick.

[mohawk2]

Thank you! Released as 0.9734.

[merkys]

Thanks a lot for merging!