Update mpl_draw() to fix multigraph plots

[maxwell04-wq]

Fixes #774. Updates:

• draw_edges(): set rad=0.25 for looped nodes.
• draw_edge_labels(): Modify label offsets for looped nodes.

[coveralls]

Pull Request Test Coverage Report for Build 9448236772

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.04%) to 95.837%

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Files with Coverage Reduction	New Missed Lines	%
rustworkx-core/src/generators/random_graph.rs	2	85.04%
src/shortest_path/all_pairs_bellman_ford.rs	6	95.53%
<!--	Total:	8		-->

Totals
Change from base Build 9447555728:	-0.04%
Covered Lines:	17380
Relevant Lines:	18135

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💛 - Coveralls

[maxwell04-wq]

@mtreinish one major hiccup in the implementation is that the position of the edges is calculated on the plot in the draw_edges() function and the position of the labels can only be evaluated relative to the position of the nodes and not the edges in the draw_edge_labels() function. While the current parameters for the curved edges of multigraphs work for plotting the labels, maybe there's a better way to evaluate the position of the labels.

[maxwell04-wq]

@IvanIsCoding I have not tried to align the labels vertically as, following the logic of rad in connectionstyle=arc3, simply offsetting the y coordinates of labels to rad generates a figure wherein the labels are perfectly placed on the edges. Would you suggest that I work on aligning the edge labels?

[IvanIsCoding]

It seems to be working, I will merge the PR