External Model State Updates

[mattldawson]

This PR adds the ability for users to access and update the portion of the solver state associated with an external model (e.g., aerosol or cloud models).

Adds square bracket operators to the State class that can be used to set elements of the State.variables_ matrix by variable index or unique name.

[Copilot]

Pull request overview

Adds square-bracket (operator[]) access to micm::State variables to support convenient external-model state reads/updates by variable index, unique name, or Species.

Changes:

• Introduced State::operator[] overloads returning proxy objects for scalar and multi-gridcell variable access/mutation.
• Added unit tests covering proxy assignment, const access, and arithmetic operations.
• Expanded aerosol-model integration tests to validate updating gas/external-model variables via the new operator[] API (including multi-cell cases).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
include/micm/solver/state.hpp	Declares operator[] overloads and proxy types for state variable access.
include/micm/solver/state.inl	Implements proxy behavior (assignment, indexing, arithmetic, comparisons) and operator[] lookups.
test/unit/solver/test_state.cpp	Adds unit tests for State::operator[] behavior in single- and multi-cell states.
test/integration/test_aerosol_model.cpp	Adds integration tests demonstrating external-model state updates via operator[] and refactors system setup into a helper.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.29%. Comparing base (5816b12) to head (d29001d).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
include/micm/solver/state.inl	79.16%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #924      +/-   ##
==========================================
- Coverage   94.63%   94.29%   -0.34%     
==========================================
  Files          68       68              
  Lines        3185     3263      +78     
==========================================
+ Hits         3014     3077      +63     
- Misses        171      186      +15     

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[K20shores]

I like it

[boulderdaze]

If this PR allows state to be manipulated by square bracket, it would effectively makes the usage of SetConcentration function obsolete. Is this a valid concern? Also I just noticed that State is struct but has public keywords. Should we consider converting State into a class and explicitly defining public/private members?

[K20shores]

We rely on SetConcentration in the wasm layer. Open to updating that if this is better though

[mattldawson]

what if we make SetConcentration deprecated, and then we can remove it as a separate issue at some point? I could also modify this PR to just overload the SetConcetration function, but this seemed a little cleaner to me.

[K20shores]

Either way is fine with me

[boulderdaze]

I like the first approach better, since it lets us evaluate the impact of removing this function on tests/tutorials, as well as on upstream including musica, atmospheric physics and cam-sima (assuming we still care about cam-sima)

[mattldawson]

sounds good! I'll add a deprecated comment to the SetConcentration fundtions

[mattldawson]

updated! I like this approach too, it will let the other repos be gradually updated before removing the SetConcentration functions.

I left the SetConcentration() function that takes a map of species to concentrations in place. This wouldn't be covered by the square bracket functions.

[boulderdaze]

Thank you for addressing the comments!