1382 Fix incorrect overload detection and type mismatch in test_commuters

[julianlitz]

Changes and Information

This PR fixes several issues preventing the correct test_commuters overload from being selected.

• Fix examples/ode_secir_graph.cpp does not use test_commuters overload #1382: The fallback empty test_commuters implementation was always selected.
• Fix Type mismatch in metaprogramming for test_commuters #1383: Removed const from Eigen::Ref<const Eigen::VectorX<FP>> in test_commuters_expr_t and corrected the default template parameter to mio::Simulation<FP, Model<FP>>.
• Fix Missing template parameter FP in the metaprogramming validation #1384: Added missing FP template parameter to is_expression_valid<test_commuters_expr_t, FP, Sim>::value.

After these changes, the correct overload is now detected and executed. Verified with ode_secir_graph.cpp where commuter testing runs as intended.

Merge Request - Guideline Checklist

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Checks by code reviewer(s)

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Closes #1382, #1383, #1384

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.25%. Comparing base (08a6cf0) to head (cb22596).

Files with missing lines	Patch %	Lines
...memilio/mobility/metapopulation_mobility_instant.h	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1394      +/-   ##
==========================================
- Coverage   97.26%   97.25%   -0.02%     
==========================================
  Files         176      176              
  Lines       15439    15446       +7     
==========================================
+ Hits        15017    15022       +5     
- Misses        422      424       +2     

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