Refactor subsolvers

[mpetrosian]

Define each subsolver in its separate module.

[codecov]

Codecov Report

Merging #400 (4bbd9c4) into main (2f968f3) will decrease coverage by 0.00%.
The diff coverage is 97.01%.

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
- Coverage   92.88%   92.87%   -0.01%     
==========================================
  Files         213      212       -1     
  Lines       17140    17111      -29     
==========================================
- Hits        15920    15892      -28     
+ Misses       1220     1219       -1     

Impacted Files	Coverage Δ
src/estimagic/optimization/subsolvers/bntr.py	87.20% <94.54%> (ø)
src/estimagic/optimization/subsolvers/gqtpar.py	90.66% <96.00%> (ø)
src/estimagic/optimization/pounders_auxiliary.py	97.33% <100.00%> (ø)
...timagic/optimization/tranquilo/solve_subproblem.py	88.46% <100.00%> (ø)
tests/optimization/test_quadratic_subsolvers.py	100.00% <100.00%> (ø)
tests/optimization/tranquilo/test_bntr_fast.py	100.00% <100.00%> (ø)

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