Refactor optimizers

[benedict-96]

Changes made:

• Adjusted constants for line search based on whether we are using single or double precision,
• fixed issues with BFGS and DFP by introducing IterativeHessian and the function initial_values_for_hessian!.

[codecov]

Codecov Report

❌ Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.06%. Comparing base (4b0b916) to head (5a1e141).

Files with missing lines	Patch %	Lines
src/linesearch/bierlaire_quadratic.jl	66.66%	2 Missing ⚠️
src/optimization/iterative_hessians.jl	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   79.67%   80.06%   +0.38%     
==========================================
  Files          42       43       +1     
  Lines        1486     1515      +29     
==========================================
+ Hits         1184     1213      +29     
  Misses        302      302              

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