change the default algorithm to the faster one in {path, cycle}_enumeration

[kappybar]

I changed the default algorithm to the faster one (and fixed the results of several tests) in

• all_simple_cycles in cycle_enumeration.py (default algorithm is changed to 'B')
• shortest_simple_paths in path_enumeration.pyx (default algorithm is changed to 'PNC')
• nc_k_shortest_simple_paths in path_enumeration.pyx (default parameter postponed is changed to True)

I found two unimplemented features in all_cycles_iterator in cycle_enumeration.py when algorithm='B'. The features are the followings:

• The algorithm 'B' is unavailable when starting_vertices is not None.
• The algorithm 'B' is unavailable when rooted=True.

I rase errors when these happen.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit daeb424; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[dcoudert]

Some doctests are failing due to these changes.
See for instance
https://github.com/sagemath/sage/actions/runs/18623771139/job/53098552404?pr=41076#step:11:1057

[dcoudert]

LGTM.