Failures down to 5 (from 7).

Added a check in get_neighbors that node_index and index should not be equal
before declaring them neighbors.
Failures down to 4 (from 5).

For the meantime, we must consider the significance of edge_count property for
graphs and how to count it properly (number of actual routes? number of
connected nodes?).
Failures down to 2 (from 4).

Fixed test for get_indegree as test was not constructing test graph before
assertions*. Additionally, the adjacency invariant in the get_indegree method
itself is wrong.

* Shouldn't we have this in the setUp method?
Removed debugging prints.