Added empty and complete graph generators

[prakharb10]

Added generators for empty and complete graphs.

Partially addresses #150.

[coveralls]

Pull Request Test Coverage Report for Build 3230698228

• 54 of 54 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 97.048%

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Files with Coverage Reduction	New Missed Lines	%
src/generators.rs	1	97.47%

Totals
Change from base Build 3221646801:	0.01%
Covered Lines:	13314
Relevant Lines:	13719

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💛 - Coveralls

[prakharb10]

Does this PR also require a reno? If yes, could you tell me what sections need to be included?

[mtreinish]

Yes I think a reno release note would be appropriate here, I would say add two notes as features, one documenting the addition of the empty graph functions and the second documenting adding the complete graph functions

[mtreinish]

Overall this LGTM, thanks for the update. I have just one small suggestion inline on the release note for complete_graph we should refer to it being the same as mesh_graph to document it's just an alias. But other than that I think this is ready to merge

[mtreinish]

I would add something here about this being identical to mesh_graph() but using a more standard name for the functionality.

[prakharb10]

Updated the reno @mtreinish. Let me know if further changes are required