[EXPERIMENTAL] Fix some graph 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]

I haven't been able to test this locally yet, but:

• Is the hasattr(buf, "replace") check necessary? If buf is a SceneBuffer, it must already have a replace() method...
• Why is replace() used instead of remove()?
• What exceptions might happen by using this? Is it OK to suppress them?

[fmuenkel]

• Yes, I believe hasattr is redundant here.
• So, this took me a while. remove() did not work - it still left Dot and Line mobjects in scene.mobjects. I just happen to try replace() and it worked, but honestly, I don't understand the new rendering process well enough yet to figure out why.
• I was thinking of cases like dry-run testing or if one of the mobjects was already removed. We don't want the run to fail, since the vertex was already created.