Update docstrings and annotations for sympy observables/sigmas

[dweindl]

Update docstrings and annotations for sympy expressions as observables/sigmas. Related to #2568.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.83%. Comparing base (e588a9f) to head (ef23d59).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2683      +/-   ##
===========================================
- Coverage    77.86%   77.83%   -0.04%     
===========================================
  Files          332      332              
  Lines        23000    23000              
  Branches      1480     1480              
===========================================
- Hits         17908    17901       -7     
- Misses        5081     5088       +7     
  Partials        11       11              

Flag	Coverage Δ
cpp	74.76% <ø> (-0.04%)	⬇️
cpp_python	33.61% <ø> (ø)
petab	39.41% <ø> (ø)
python	72.86% <ø> (-0.03%)	⬇️

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Files with missing lines	Coverage Δ
python/sdist/amici/sbml_import.py	79.32% <ø> (ø)

... and 2 files with indirect coverage changes

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