Refactor: move Model::state_ initalization to ModelState ctor

[dweindl]

Move Model::state_ initalization to ModelState ctor where it belongs.

Closes #2531.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.79%. Comparing base (b7a3e91) to head (5d4bbb8).
Report is 80 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2533   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files          325      325           
  Lines        21053    21057    +4     
  Branches      1473     1473           
========================================
+ Hits         16378    16382    +4     
  Misses        4648     4648           
  Partials        27       27           

Flag	Coverage Δ
cpp	73.51% <100.00%> (+<0.01%)	⬆️
cpp_python	34.05% <100.00%> (+0.01%)	⬆️
petab	37.05% <100.00%> (+0.01%)	⬆️
python	72.34% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
include/amici/model_state.h	78.26% <100.00%> (+1.40%)	⬆️
src/model.cpp	84.44% <100.00%> (-0.03%)	⬇️