networkx

Description

Currently our unit tests are disorganized and each test creates example StellarGraph graphs in different or similar ways with no sharing of this code.

This issue is to improve the unit tests by making functions to create example graphs available to all unit tests by, for example, making them pytest fixtures at the top level of the tests (see https://docs.pytest.org/en/latest/

Let's show some examples of integration with kgextension
https://kgextension.readthedocs.io/en/latest/

Could be another notebook added to the tutorial.

Where it fits, we might also integrate as a dependency?