OPT: Combine cost/gradient into one

[rcjackson]

I am currently trying to speed up PyDDA. The first optimization:

• The cost function and gradient are now only called once on each iteration and combined.

Co-written by GPT-5.6 Luna

[codecov]

Codecov Report

❌ Patch coverage is 91.60305% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.56%. Comparing base (6572b81) to head (b3c3ec8).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pydda/cost_functions/cost_functions.py	81.96%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
- Coverage   70.57%   70.56%   -0.02%     
==========================================
  Files          32       32              
  Lines        5673     5793     +120     
==========================================
+ Hits         4004     4088      +84     
- Misses       1669     1705      +36     

Flag	Coverage Δ
unittests	70.56% <91.60%> (-0.02%)	⬇️

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