Proper error message in case of unsupported state-dependent sigmas

[dweindl]

State-dependent sigmas are currently not supported and result in compiler errors.

Catch them before compilation and quit with a more informative error message.

Closes #919

[codecov]

Codecov Report

Merging #2239 (fb0169b) into develop (664994c) will increase coverage by 0.14%.
Report is 23 commits behind head on develop.
The diff coverage is 81.23%.

❗ Current head fb0169b differs from pull request most recent head ecba04c. Consider uploading reports for the commit ecba04c to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2239      +/-   ##
===========================================
+ Coverage    76.51%   76.65%   +0.14%     
===========================================
  Files           86       91       +5     
  Lines        15025    15008      -17     
===========================================
+ Hits         11496    11505       +9     
+ Misses        3529     3503      -26     

Flag	Coverage Δ
cpp	73.11% <83.80%> (+0.03%)	⬆️
cpp_python	37.08% <58.09%> (+0.04%)	⬆️
petab	53.78% <75.97%> (+0.03%)	⬆️
python	78.01% <51.58%> (+0.85%)	⬆️
sbmlsuite	∅ <ø> (∅)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
include/amici/forwardproblem.h	100.00% <100.00%> (ø)
include/amici/misc.h	64.00% <ø> (ø)
include/amici/model.h	61.11% <ø> (ø)
include/amici/model_dae.h	0.00% <ø> (ø)
include/amici/steadystateproblem.h	76.47% <ø> (ø)
python/sdist/amici/__main__.py	0.00% <ø> (ø)
python/sdist/amici/de_model.py	93.28% <100.00%> (+0.42%)	⬆️
python/sdist/amici/import_utils.py	88.23% <100.00%> (+0.28%)	⬆️
python/sdist/amici/petab/cli/import_petab.py	0.00% <ø> (ø)
python/sdist/amici/petab/simulator.py	100.00% <100.00%> (ø)
... and 30 more

... and 3 files with indirect coverage changes