Bug fix for polynomial optimization for PERK integrator

[warisa-r]

When the user used the integrator ode_algorithm = Trixi.PairedExplicitRK2(3, tspan, semi), gamma_opt from Trixi.bisect_stability_polynomial would be returned as a number, which caused a set_index error when Trixi.undo_normalization! was called after in methods_PERK2.

This fix attempts to address this issue effectively without making major changes to the existing code.

[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.22%. Comparing base (2d0aa80) to head (78e2585).

Files	Patch %	Lines
ext/TrixiConvexECOSExt.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2004      +/-   ##
==========================================
- Coverage   96.22%   96.22%   -0.00%     
==========================================
  Files         462      462              
  Lines       37043    37046       +3     
==========================================
+ Hits        35644    35646       +2     
- Misses       1399     1400       +1     

Flag	Coverage Δ
unittests	96.22% <75.00%> (-<0.01%)	⬇️

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[DanielDoehring]

Good catch, thanks!