Zb/illico threads

[zboldyga]

• Closes #
• [x ] [Tests][x] included or not required because:

• Release notes not necessary because:

@ilan-gold this is a small PR that does a few things:

• illico previously wasn't getting a thread value passed down (e.g. n_jobs), it was defaulting to 1. that is fixed
• there was a preexisting bug with the _numba utility for resolving thread count, it resolved -1 to 1 thread. the only place it was used was in rank_genes_groups, which passed n_jobs, so this conflicted with scanpy docs. This now properly sets to the numba thread count
• we also wrap the illico call with the _numba utility so numba thread count restores upon completion (I think illico modifies numba thread count, this helps prevent side effects).

[zboldyga]

@ilan-gold while not high impact, I also saw some inconsistencies with the way n_jobs is used throughout scanpy. I preferred not to open that can of worms here. Might just be something that is most easily solved with clear docs in each use case anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.88%. Comparing base (eea0015) to head (d2f7004).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4205      +/-   ##
==========================================
- Coverage   79.90%   79.88%   -0.02%     
==========================================
  Files         121      121              
  Lines       12949    12958       +9     
==========================================
+ Hits        10347    10352       +5     
- Misses       2602     2606       +4     

Flag	Coverage Δ
hatch-test.low-vers	?
hatch-test.pre	79.88% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/scanpy/_utils/_numba.py	100.00% <100.00%> (ø)
src/scanpy/tools/_rank_genes_groups.py	94.16% <100.00%> (ø)

... and 3 files with indirect coverage changes