PEtab: Fix estimated initial conditions specified via the conditions table

[dweindl]

• For reinitialization of state variables after preequilibration, we need to add fixed parameters for initial conditions (otherwise parameter values can't be different for pre-equilibration and the following period). This case was handled fine.
• If we want to estimate initial conditions and want to compute sensitivities w.r.t. those, we need non-fixed parameters for the initial conditions. This was not handled correctly, those were added as fixed parameters.
• As a consequence, we can't handle reinitialization and estimation of initial values for the same state variable.

Closes #2455.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.65%. Comparing base (2b8c6a2) to head (531a2a5).
Report is 30 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2456   +/-   ##
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  Coverage    77.64%   77.65%           
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  Files          324      324           
  Lines        20927    20927           
  Branches      1464     1464           
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+ Hits         16248    16250    +2     
+ Misses        4676     4674    -2     
  Partials         3        3           

Flag	Coverage Δ
cpp	73.41% <ø> (ø)
cpp_python	34.09% <ø> (ø)
petab	36.66% <ø> (+0.01%)	⬆️
python	72.19% <ø> (ø)

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Files with missing lines	Coverage Δ
python/sdist/amici/petab/cli/import_petab.py	0.00% <ø> (ø)
python/sdist/amici/petab/import_helpers.py	86.15% <ø> (+3.07%)	⬆️
python/sdist/amici/petab/sbml_import.py	54.28% <ø> (ø)