DM-38418: Override get method in BpsConfig to make default value parameter work

[mxk62]

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: +0.12 🎉

Comparison is base (b2e9cf8) 78.43% compared to head (f028e0a) 78.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   78.43%   78.55%   +0.12%     
==========================================
  Files          39       39              
  Lines        2856     2872      +16     
  Branches      490      490              
==========================================
+ Hits         2240     2256      +16     
  Misses        540      540              
  Partials       76       76              

Impacted Files	Coverage Δ
python/lsst/ctrl/bps/bps_config.py	94.77% <100.00%> (+0.11%)	⬆️
tests/test_bpsconfig.py	98.23% <100.00%> (+0.23%)	⬆️

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[MichelleGower]

Implementation matches expectations. Just two minor comments about a docstring and some comments about unit tests. Merge approved.