Improve subsolver BNTR

[segsell]

In this PR, we improve the conjugate gradient step of the BNTR (Bounded Newton Trust-Region) algorithm by adding two conjugate gradient methods.
Previously, the only available option was an off-the-shelf conjugate gradient implementation.

The new methods are:

• Steihaug-Toint (see Steihaug, 1983)
• TRSBOX (see Powell, 2009)

The respective flags for our three methods are:

• "cg"
• "steihaug_toint"
• "trsbox"

TRSBOX performs best on the more_wild benchmark set and is thus the recommended default.

[codecov]

Codecov Report

Merging #328 (9d9f4fa) into main (6b4576d) will decrease coverage by 0.15%.
The diff coverage is 87.47%.

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
- Coverage   90.85%   90.70%   -0.16%     
==========================================
  Files         171      173       +2     
  Lines       12439    12825     +386     
==========================================
+ Hits        11302    11633     +331     
- Misses       1137     1192      +55     

Impacted Files	Coverage Δ
...ptimization/subsolvers/bounded_newton_quadratic.py	85.96% <79.16%> (ø)
...imagic/optimization/subsolvers/gqtpar_quadratic.py	89.31% <80.95%> (ø)
...magic/optimization/subsolvers/_trsbox_quadratic.py	84.08% <84.08%> (ø)
...timization/subsolvers/_steihaug_toint_quadratic.py	89.88% <89.88%> (ø)
src/estimagic/optimization/pounders_auxiliary.py	97.37% <95.23%> (+0.05%)	⬆️
src/estimagic/optimization/pounders.py	93.12% <100.00%> (ø)
...zation/subsolvers/_conjugate_gradient_quadratic.py	100.00% <100.00%> (ø)
...magic/optimization/subsolvers/linear_subsolvers.py	95.94% <100.00%> (ø)
...ic/optimization/subsolvers/quadratic_subsolvers.py	95.83% <100.00%> (ø)
tests/optimization/test_linear_subsolvers.py	100.00% <100.00%> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6b4576d...9d9f4fa. Read the comment docs.

[janosg]

Thanks a lot! This is very important for pounders and tranquilo!