creation of nested subgraphs by vertex descriptors is broken/unintuitive

When using `subgraph::create_subgraph(VertexIterator first, VertexIterator last)`, the vertex descriptors are interpreted as vertex descriptors into the root graph. (`add_vertex` expects the given descriptor to be a root graph descriptor).

My expectation is that the vertex descriptors should be interpreted as descriptors for the subgraph on which `create_subgraph` is called on. This way you can get a subgraph object, do some vertex selection logic on it and pass the set of selected vertices to `create_subgraph` without ever needing to worry where the graph is situated in the subgraph tree.

The fix is to replace line 197 `add_vertex(*first, *m_children.back());` with `add_vertex(local_to_global(*first), *m_children.back());`.