add bfs_predecessors

[ewinston]

This adds a bfs_predecessors method to complement bfs_successors.

[coveralls]

Pull Request Test Coverage Report for Build 4619289530

• 69 of 79 (87.34%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 97.072%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/iterators.rs	35	45	77.78%

Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	1	98.88%
src/shortest_path/all_pairs_dijkstra.rs	1	98.54%

Totals
Change from base Build 4580675812:	-0.07%
Covered Lines:	14092
Relevant Lines:	14517

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💛 - Coveralls

[mtreinish]

Overall this LGTM, it's pretty straightforward just a copy of the bfs_successors code with the direction reversed. I left one inline comment about an improvement we can make (which is also something we should do to bfs_successors).

Besides that the only thing that I think is missing is a release note for the new function.

[mtreinish]

This LGTM now, thanks for the updates. I just left an inline suggestion on the release note text. Also I just noticed bfs_predecessors is not in the api documentation, it just needs to be added in: https://github.com/Qiskit/rustworkx/blob/main/docs/source/api.rst to the traversal section.

[mtreinish]

LGTM, thanks for making the updates.