DM-39434: Move QBB factory to a separate class

[andy-slac]

It was a nested method before and that interfered with pickle in multiprocessing on MacOS. Moved that to a separate callable class.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Patch coverage: 44.44% and project coverage change: +1.04 🎉

Comparison is base (572e431) 84.84% compared to head (8b1e934) 85.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   84.84%   85.88%   +1.04%     
==========================================
  Files          50       50              
  Lines        4552     4500      -52     
  Branches      814      793      -21     
==========================================
+ Hits         3862     3865       +3     
+ Misses        502      468      -34     
+ Partials      188      167      -21     

Impacted Files	Coverage Δ
python/lsst/ctrl/mpexec/cmdLineFwk.py	62.74% <44.44%> (+0.77%)	⬆️

... and 20 files with indirect coverage changes

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[timj]

Is it possible to pickle something in ctrl_mpexec to trigger this code?

[andy-slac]

Is it possible to pickle something in ctrl_mpexec to trigger this code?

I can't think of anything right now, but pipelines_check will test it now.