Add g4org export options and ORANGE stats

[sethrj]

This implements pydantic models for celeritas-project/celeritas#2018 and celeritas-project/celeritas#2036 .

[Copilot]

Pull Request Overview

This PR refactors the Celeritas Python model structure by splitting a monolithic model.py file into modular submodules (types.py, input.py, output.py), while adding support for ORANGE statistics and Geant4-to-ORANGE conversion options.

Key changes:

• Split model.py into model/types.py, model/input.py, and model/output.py for better organization
• Added OrangeConversionOptions, OrangeStats, and related output models (BihSizes, OrangeParamsOutput, etc.)
• Updated settings to support g4org_options (JSON file path) and g4_geo_optimize instead of deprecated g4org_verbose and g4org_export

Reviewed Changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
celerpy/model.py	Converted to a backward-compatibility wrapper that re-exports from new submodules
celerpy/model/__init__.py	New module init providing re-exports for backward compatibility
celerpy/model/types.py	Extracted base types, enums (GeometryEngine, MemSpace, UnitSystem, LogLevel), and Tolerance model
celerpy/model/input.py	Extracted input models including new OrangeConversionOptions, OrangeStats, and TraceSetup with _cmd field
celerpy/model/output.py	Extracted output models including new ORANGE-related models (BihSizes, OrangeParamsOutput, etc.)
celerpy/visualize.py	Updated imports to use new modular structure
celerpy/process.py	Updated imports and fixed iteration over settings object
celerpy/conf/settings.py	Replaced g4org_verbose and g4org_export with g4org_options and g4_geo_optimize
test/test_model.py	Updated tests for new module structure and added tests for OrangeStats and BihSizes
test/test_visualize.py	Updated imports and added assertion for masked array type
test/data/options.json	Added empty JSON file for testing
README.md	Minor wording correction

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