[EXPERIMENTAL] Fix test_graph.py tests

[fmuenkel]

Overview: What does this pull request change?

Currently a lot of tests in test_graph.py fail, because __str__() is missing from Graph and DiGraph.
This PR

• adds __str__() wrapping around __repr__()
• uses numpy's assert_allclose() to make two more test pass
• fixes a bug in graph.py's GenericGraph._add_vertices_animation() where not all temporary vertices were removed from the scenebuffer after the animation.

Now all tests in test_graph.py pass.

Motivation and Explanation: Why and how do your changes improve the library?

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

[chopan050]

Thanks! I left some comments:

[chopan050]

LGTM! See my latest comment for more information on what's going on in GenericGraph._add_vertices_animation() in the end.