Graph format of tx endpoint incorporates nodes/rels from paths.

Implemented by having GraphExtractionWriter recognize Path objects, and
add the Nodes and Relationships of the Path to the extraction.
The Nodes of the Relationships of the Path are not added, since they are
part of the path already.

As part of this change a set of convenience methods for mocking Node,
Relationship, and Path, called "GraphMock" was introduced, and used in
a few places where custom mocking of these types was employed before.