DM-39605: Use butler.dimensions rather than butler.registry.dimensions

[timj]

Checklist

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

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.26 ⚠️

Comparison is base (fcff66f) 85.70% compared to head (c97bf03) 85.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
- Coverage   85.70%   85.45%   -0.26%     
==========================================
  Files          47       47              
  Lines        4198     4200       +2     
  Branches      722      722              
==========================================
- Hits         3598     3589       -9     
- Misses        448      458      +10     
- Partials      152      153       +1     

Impacted Files	Coverage Δ
python/lsst/ctrl/mpexec/cli/cmd/commands.py	66.22% <ø> (ø)
python/lsst/ctrl/mpexec/cli/opt/optionGroups.py	100.00% <ø> (ø)
python/lsst/ctrl/mpexec/cli/pipetask.py	93.33% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/build.py	81.81% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/cleanup.py	96.87% <ø> (ø)
...n/lsst/ctrl/mpexec/cli/script/pre_exec_init_qbb.py	42.85% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/qgraph.py	42.85% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/run.py	45.45% <ø> (ø)
python/lsst/ctrl/mpexec/cli/script/run_qbb.py	42.85% <ø> (ø)
python/lsst/ctrl/mpexec/cli/utils.py	90.00% <ø> (ø)
... and 17 more

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[timj]

@andy-slac here's another large PR for docstrings and type annotations. Sorry. Can wait until next week.

[andy-slac]

Looks great, few minor comments.