Fix substitution of expressions in drootdt_total

[dilpath]

Not sure if this is a recent error, or if SymPy never used the dictionary order, but this fixes an error for a student's model where w symbols appeared in the generated C++ for stau.

[codecov]

Codecov Report

Merging #1512 (a19d968) into develop (39aad8d) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1512      +/-   ##
===========================================
+ Coverage    79.39%   79.46%   +0.07%     
===========================================
  Files           67       67              
  Lines        10399    10399              
===========================================
+ Hits          8256     8264       +8     
+ Misses        2143     2135       -8     

Flag	Coverage Δ
cpp	75.88% <ø> (+0.11%)	⬆️
petab	67.66% <0.00%> (ø)
python	68.25% <100.00%> (ø)
sbmlsuite	88.06% <0.00%> (ø)

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Impacted Files	Coverage Δ
python/amici/ode_export.py	93.15% <100.00%> (ø)
src/exception.cpp	82.60% <0.00%> (ø)
src/solver_cvodes.cpp	70.20% <0.00%> (ø)
src/sundials_matrix_wrapper.cpp	84.21% <0.00%> (+0.42%)	⬆️
src/amici.cpp	75.65% <0.00%> (+5.21%)	⬆️

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

100.0% Coverage
0.0% Duplication