Free allocated space for vertices when graph is destroyed while using WeightedGraphAspect

```
Space allocated for vertices should be freed when graph is destroyed while
using WeightedGraphAspect.
```

Original issue reported on code.google.com by `bombax` on 8 Feb 2012 at 8:28

Start by locating WeightedGraphAspect and the graph destruction path in the C++ source. Trace how vertex space is allocated and released, then verify that destroying a graph using the aspect leaves no vertex allocation outstanding; the issue names no test, so add or run the project's relevant graph-lifecycle checks if available.

Written by the indexing model from the issue text.