ReadOnlyGraphAggregate.quads should accept a quad

This part is wrong:

```
   1830     def quads(self, (s, p, o)):
   1831         """Iterate over all the quads in the entire aggregate graph"""
   1832         for graph in self.graphs:
   1833             for s1, p1, o1 in graph.triples((s, p, o)):
   1834                 yield (s1, p1, o1, graph)
```

It should accept a context in the incoming tuple.
