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 13579523991

Details

• 61 of 61 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 95.815%

---

Totals
Change from base Build 13320858474:	0.5%
Covered Lines:	18547
Relevant Lines:	19357

---

💛 - 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