Correct constant-species index mapping in SimpleReactor

[alongd]

Motivation or Problem

SimpleReactor.get_const_spc_indices() was possibly effectively broken:

• It reused the same loop variable name in nested loops.
• It compared spc.label == spc, i.e. a string to a species object.

As a result, const_spc_indices was never populated and species listed in const_spc_names were perhaps not actually treated as constant.

Description of Changes

In SimpleReactor.get_const_spc_indices():

• Use different variable names for const_spc_names and core_species.
• Compare the constant-species name (string) against spc.label instead of spc itself.
• Initialize const_spc_indices once and append core-species indices when labels match.
• Return early if const_spc_names is None.

Testing

A small unit test was added.

Reviewer Tips

In a small reactor run with a constant species, check that its amount stays fixed over time.

[Copilot]

Pull request overview

This PR fixes a critical bug in SimpleReactor.get_const_spc_indices() that prevented constant species from being properly identified and treated as constant during simulation. The original implementation had two major issues: it reused the same loop variable spc in nested loops, and it incorrectly compared a string to a Species object (spc.label == spc instead of spc.label == name). As a result, species listed in const_spc_names were never actually marked as constant.

• Fixed variable shadowing in nested loops by using distinct variable names (name for const_spc_names, spc for core_species)
• Corrected the comparison logic to properly match species labels against constant species names
• Added early return for None const_spc_names and moved initialization outside the loop for clarity
• Added a break statement to stop searching once a match is found

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rmgpy/solver/simple.pyx	Fixed the get_const_spc_indices() method by correcting variable naming, comparison logic, and control flow
test/rmgpy/solver/simpleTest.py	Added unit test test_get_const_spc_indices() to verify that constant species names are correctly mapped to core species indices

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[alongd]

A test unrelated to this PR failed:

FAILED test/rmgpy/reactionTest.py::TestReactionToCantera::test_arrhenius - AssertionError: assert {'rate-constant': {'A': 660000.0, 'b': 1.62, 'Ea': 45354560.0}} == {'rate-constant': {'A': 660000.0000000001, 'b': 1.62, 'Ea': 45354560.0}}
  
  Differing items:
  {'rate-constant': {'A': 660000.0, 'Ea': 45354560.0, 'b': 1.62}} != {'rate-constant': {'A': 660000.0000000001, 'Ea': 45354560.0, 'b': 1.62}}
  
  Full diff:
    {
        'rate-constant': {
  -         'A': 660000.0000000001,
  ?                      ---------
  +         'A': 660000.0,
            'Ea': 45354560.0,
            'b': 1.62,
        },
    }
FAILED test/rmgpy/reactionTest.py::TestReactionToCantera::test_multi_arrhenius - assert 0.001 == 0
 +  where 0.001 = round(0.0009765625, 3)
 +    where 0.0009765625 = abs((5000000000000.0 - 5000000000000.001))
 +      where 5000000000000.0 = <ArrheniusRate at 7f9319b52cf0>.pre_exponential_factor
 +        where <ArrheniusRate at 7f9319b52cf0> = HO2(5) + OH(4) <=> H2O(27) + O2(6)    <Reaction(Arrhenius)>.rate
 +      and   5000000000000.001 = <ArrheniusRate at 7f9319b529f0>.pre_exponential_factor
 +        where <ArrheniusRate at 7f9319b529f0> = HO2(5) + OH(4) <=> H2O(27) + O2(6)    <Reaction(Arrhenius)>.rate
FAILED test/rmgpy/reactionTest.py::TestChargeTransferReaction::test_get_rate_coeff - assert 0.0078125 < 1e-06
 +  where 0.0078125 = abs((43870506959778.99 - 43870506959779.0))
== 3 failed, 1968 passed, 39 skipped, 3939699 warnings in 2192.81s (0:36:32) ===

Added another commit to check isclose() for 660000.0000000001 vs. 660000.0 instead of identity