refactor: 0-based indexing in Model::z2event_

[dweindl]

Get rid of 1-based event-indexing in Model::z2event_ from ancient MATLAB times.

Related to #2727.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.60%. Comparing base (d5492c3) to head (fda9b48).
⚠️ Report is 125 commits behind head on main.

Files with missing lines	Patch %	Lines
python/sdist/amici/de_model.py	0.00%	3 Missing ⚠️
models/model_calvetti_py/model_calvetti_py.h	0.00%	1 Missing ⚠️
models/model_dirac_py/model_dirac_py.h	0.00%	1 Missing ⚠️
models/model_events_py/model_events_py.h	50.00%	1 Missing ⚠️
...odel_jakstat_adjoint_py/model_jakstat_adjoint_py.h	0.00%	1 Missing ⚠️
...ls/model_nested_events_py/model_nested_events_py.h	0.00%	1 Missing ⚠️
models/model_neuron_py/model_neuron_py.h	50.00%	1 Missing ⚠️
models/model_robertson_py/model_robertson_py.h	0.00%	1 Missing ⚠️
models/model_steadystate_py/model_steadystate_py.h	0.00%	1 Missing ⚠️
src/model.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2964      +/-   ##
==========================================
- Coverage   77.29%   76.60%   -0.70%     
==========================================
  Files         307      307              
  Lines       19912    19912              
  Branches     1502     1502              
==========================================
- Hits        15391    15253     -138     
- Misses       4508     4646     +138     
  Partials       13       13              

Flag	Coverage Δ
cpp	72.75% <33.33%> (-0.02%)	⬇️
cpp_python	38.30% <0.00%> (ø)
petab	39.36% <0.00%> (ø)
python	70.32% <33.33%> (-0.02%)	⬇️
sbmlsuite-jax	?

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

Files with missing lines	Coverage Δ
models/model_calvetti_py/Jy.cpp	0.00% <ø> (ø)
models/model_calvetti_py/create_splines.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dJydsigma.cpp	0.00% <ø> (ø)
models/model_calvetti_py/dJydy.cpp	0.00% <ø> (ø)
models/model_calvetti_py/dwdw.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dwdx.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dxdotdw.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dxdotdx_explicit.cpp	100.00% <ø> (ø)
models/model_calvetti_py/dydx.cpp	0.00% <ø> (ø)
models/model_calvetti_py/explicit_roots.cpp	100.00% <ø> (ø)
... and 173 more

... and 8 files with indirect coverage changes

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