Add direct versions of 4 generators

[enavarro51]

Fixes #751.

This PR adds directed versions of the lollipop, barbell, full_rary_tree, and petersen generators. It's based on #784, and should be merged after that one.

[coveralls]

Pull Request Test Coverage Report for Build 3925466615

• 302 of 413 (73.12%) changed or added relevant lines in 6 files are covered.
• 44 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.007%) to 95.942%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
rustworkx-core/src/generators/full_rary_tree_graph.rs	39	40	97.5%
rustworkx-core/src/generators/lollipop_graph.rs	11	18	61.11%
rustworkx-core/src/generators/petersen_graph.rs	7	18	38.89%
rustworkx-core/src/generators/barbell_graph.rs	14	27	51.85%
src/generators.rs	217	296	73.31%

Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	1	98.88%
src/generators.rs	43	82.97%

Totals
Change from base Build 3916313470:	-1.007%
Covered Lines:	13596
Relevant Lines:	14171

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💛 - Coveralls

[IvanIsCoding]

I am against this getting merged because the directed version of some of the graphs you proposed are not common or useful. I will take for example the Generalized Petersen graph, but it also applies to the others.

By definition the graph is undirected. Almost all of the literature about the graph is about it being undirected and its properties that are related to that. Why would we generate a graph that people don't use? It misses the point of why generators exist.

If someone ever needs the topology of some of the graphs and needs to change from undirected to directed, they can call to_directed() and it works. So I'd rather not merge this

[enavarro51]

Thanks for the feedback, Ivan. I was only doing it for consistency reasons and if they're not needed, best not to add them. I'll close this.