Add keyword arguments pre_callback and post_callback to optimization problems

[sblauth]

This PR adds the keyword arguments "pre_callback" and "post_callback" to optimization problems, which allows users to define the callbacks directly when instanciating the optimization problem.

[codecov]

Codecov Report

Merging #248 (acce0d0) into main (2713467) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #248   +/-   ##
=======================================
  Coverage   92.58%   92.58%           
=======================================
  Files          83       83           
  Lines        6442     6444    +2     
=======================================
+ Hits         5964     5966    +2     
  Misses        478      478           

Impacted Files	Coverage Δ
...n/shape_optimization/shape_optimization_problem.py	95.04% <ø> (ø)
...logy_optimization/topology_optimization_problem.py	91.03% <ø> (ø)
...ization/optimal_control/optimal_control_problem.py	98.13% <100.00%> (ø)
cashocs/_optimization/optimization_problem.py	94.30% <100.00%> (+0.07%)	⬆️

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