Add parallelism to cloness_centrality

[IvanIsCoding]

This PR makes closeness_centrality and newman_weighted_closeness_centrality parallel for better performance. This is possible because the task is embarrassingly parallel.

We probably missed this in #593 but #1385 reminded me of it.

The parallelism is toggled via the parallel_threshold just like other centrality methods.

[coveralls]

Pull Request Test Coverage Report for Build 13946980567

Details

• 89 of 89 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 95.825%

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Totals
Change from base Build 13938071754:	0.5%
Covered Lines:	18589
Relevant Lines:	19399

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💛 - Coveralls

[IvanIsCoding]

Because #1387, I added a test to boost the coverage in the report. I am confident the Rust tests guarantee the correctness so for now that is a stop gap

[mtreinish]

This LGTM, it makes a lot of sense to run this with rayon. Thanks for doing this. I just left a couple of comments inline the main one is I think we can do this without a mutex using a rayon iterator collection into the vec which should be faster..