Clean up Params.ini, handle m2_min=None

[TomWagg]

This pull request updates the parameter documentation and configuration options for binary population synthesis in the examples/Params.ini file and the associated UI schema in src/cosmic/data/cosmic-settings.json. The changes clarify option descriptions, add new models and parameters, and improve consistency across documentation and settings.

Key changes include:

New Models and Parameters:

• Added the martinez26 model option for porb_model, which uses sana12 for massive binaries (≥8 M☉) and raghavan10 for lower-mass binaries, as used in Martinez+2026.
• Introduced a new parameter, keep_singles, to control whether single stars are retained in the sampled population, with documentation and UI support. [1] [2]

Clarifications and Consistency Improvements:

• Clarified that only one of qmin or m2_min should be specified as non-default (not None or 0), and updated option descriptions accordingly in both the INI and JSON files. [1] [2] [3] [4]
• Updated several option descriptions to use consistent terminology (e.g., "range [0, 1]" instead of "values between [0, 1]") and clarified allowable value ranges for parameters like hewind, xi, bhsigmafrac, rembar_massloss, epsnov, rejuv_fac, and others. [1] [2] [3] [4] [5] [6] [7]

Model and Option Updates:

• Changed offner22 to offner23 for the mass-dependent binary fraction model.
• Added new negative values for kickflag to allow use of the old Kiel & Hurley 2009 prescription for binary orbital changes, and set the default to 5.
• Updated the default for maltsev_mode to 0 and expanded documentation for Maltsev remnant mass prescription parameters.
• Improved descriptions for several other parameters, including new explicit options for ignoring or disabling certain effects (e.g., gamma = 0.0 for no angular momentum loss, acc_lim = 0.0 for fully nonconservative mass transfer). [1] [2]

Formatting and Syntax Corrections:

• Standardized example dictionary formatting in convergence_limits to use double quotes for JSON compatibility. [1] [2]
• Updated option descriptions for clarity and accuracy throughout the configuration files. (multiple references above)

These changes improve the usability, clarity, and flexibility of the configuration and documentation for users and developers.

[codecov]

Codecov Report

❌ Patch coverage is 68.18182% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.82%. Comparing base (8772c07) to head (8b7efb9).
⚠️ Report is 133 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/cosmic/tests/test_sample.py	78.95%	4 Missing ⚠️
src/cosmic/sample/sampler/independent.py	0.00%	2 Missing ⚠️
src/cosmic/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #749      +/-   ##
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- Coverage    86.91%   77.82%   -9.09%     
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  Files           40       50      +10     
  Lines        25542    27456    +1914     
  Branches         0      920     +920     
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- Hits         22198    21365     -833     
- Misses        3344     5817    +2473     
- Partials         0      274     +274     

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