Fix check for sign change, fix Heaviside update

[dweindl]

The old check for sign changes did not work if one values was zero.

Heaviside state has to be updated independent of whether the event occurs at the initial timepoint or not.

Fixes #2926.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.01%. Comparing base (9be5026) to head (e64ef5d).
⚠️ Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2927      +/-   ##
==========================================
- Coverage   79.57%   79.01%   -0.57%     
==========================================
  Files         362      362              
  Lines       23156    23157       +1     
  Branches     1589     1588       -1     
==========================================
- Hits        18427    18297     -130     
- Misses       4717     4848     +131     
  Partials       12       12              

Flag	Coverage Δ
cpp	74.94% <100.00%> (+<0.01%)	⬆️
cpp_python	33.12% <50.00%> (+<0.01%)	⬆️
petab	38.78% <0.00%> (-0.01%)	⬇️
python	73.80% <100.00%> (+<0.01%)	⬆️
sbmlsuite-jax	?

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Files with missing lines	Coverage Δ
include/amici/misc.h	91.30% <100.00%> (+0.39%)	⬆️
src/forwardproblem.cpp	90.60% <100.00%> (-0.02%)	⬇️

... and 6 files with indirect coverage changes

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