At the moment an edge requires both vertices to be of the same namespace.
However only one of both vertices must be.

In order to implement this properly the edge can no longer rely on two vertices being present as one side of the relation may not be available in the current graph (or graph container) or at all.

Each edge should only reference the `VertexRef` objects and no longer use `Vertex`, but should still ensure that one side of the relationship is within the same namespace as the edge itself.

Please keep in mind, that this also has an effect on the persistence layer, as at the moment "loose" references are not supported (due to foreign key constraints).