Semiquant: Add spline knots to the optimization result

[Doresic]

Fixes #1308 by saving the spline knots in the OptimizerResult object under spline_knots in the form of list[list[np.array[float], np.array[float]]]. Each list in the first level corresponds to a semiquantitative observable. Each of these lists contains two arrays: the first array contains the spline bases, the second array contains the spline knot values. The ordering of the observable lists is the same as in pypesto.problem.hierarchical.semiquant_observable_ids.

This also allows the visualization plot_splines_from_pypesto_result to just extract the spline knot values from the pypesto result object instead of solving the inner problem to get them. The latter could even a bit wrong due to possibly different initial conditions of the inner optimization problem.

Except this, there is some random cleanup here and there.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 84.44%. Comparing base (7ecee81) to head (ffa7c26).

Files	Patch %	Lines
pypesto/visualize/spline_approximation.py	0.00%	6 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1314      +/-   ##
===========================================
- Coverage    84.48%   84.44%   -0.04%     
===========================================
  Files          153      153              
  Lines        12455    12478      +23     
===========================================
+ Hits         10522    10537      +15     
- Misses        1933     1941       +8     

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[FFroehlich]

only looked at changes in pypesto/objective/amici/amici.py

[PaulJonasJost]

Do we have a test yet, saving and loading this results? If not, please add before merging. Otherwise looks good to me

[Doresic]

Do we have a test yet, saving and loading this results? If not, please add before merging. Otherwise looks good to me

Added a test to test/hierarchical/test_spline.py
I'm just running a simple optimization with 2 starts, saving the result, loading it, and checking the spline knots before and after are the same.

[m-philipps]

I don't see any issues in the visualize/spline_approximation.py. For the future it would be useful to have you as a codeowner of that file.