OPT: Disable SciPy parallel option

[rcjackson]

Disabling the SciPy parallel option as it only showed a 3% performance gain on a 12-core machine. Assisted by GPT-5.6 Luna.

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.42%. Comparing base (50e7230) to head (d62348d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pydda/cost_functions/cost_functions.py	54.54%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
- Coverage   70.43%   70.42%   -0.02%     
==========================================
  Files          32       32              
  Lines        5865     5856       -9     
==========================================
- Hits         4131     4124       -7     
+ Misses       1734     1732       -2     

Flag	Coverage Δ
unittests	70.42% <54.54%> (-0.02%)	⬇️

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