Move core_number to rustworkx-core

[enavarro51]

This PR moves the function core_number from src/connectivity/core_number.rs to rustworkx-core/connectivity/core_number.rs.

[coveralls]

Pull Request Test Coverage Report for Build 4079118655

• 68 of 68 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 97.081%

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Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	2	98.88%

Totals
Change from base Build 4075927635:	0.03%
Covered Lines:	13736
Relevant Lines:	14149

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💛 - Coveralls

[enavarro51]

There was a cut and paste docs error in core_number which I also found in find_cycle and fixed both here.

[IvanIsCoding]

I left some minor comments. Most of them are easy to address. The harder things are:

• Should be remove the parallel sorting of edges? I am not as confident about that
• We probably should handle the invalid inputs now that we are changing the code

[IvanIsCoding]

Food for thought: should we remove the parallel sort? It might be worth benchmarking for large graphs

[enavarro51]

No opinion on it. I'll try a quick benchmark and see if it shows a difference.

[enavarro51]

Ran it with heavy hex with low core number and complete with high core number on my local and there was less that 5% variation between sort_by_key and par_sort_by_key. Maybe for some other types of graphs it could be different.

[IvanIsCoding]

I’d import Rayon and use par_sort_by_key. Rayon is in the dependencies already, might as well use it as the previous code for consistency

[enavarro51]

Ok. I'll put par_sort_by_key back.

[enavarro51]

Ok. I'll leave the implicitly assumes for later and will open an issue for it.