Re-add mistakenly removed method for loading a config from file

[FBumann]

Description

Re-add mistakenly removed method for loading a config from file

Type of Change

• Bug fix
• New feature
• Documentation update
• Code refactoring

Related Issues

Closes #(issue number)

Testing

• I have tested my changes
• Existing tests still pass

Checklist

• My code follows the project style
• I have updated documentation if needed
• I have added tests for new functionality (if applicable)

Summary by CodeRabbit

• New Features
  • Configuration can now be loaded from YAML files.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Caution

Review failed

The pull request is closed.

Walkthrough

This PR re-adds the CONFIG.load_from_file() method, which reads YAML configuration files, validates existence, and applies parsed configuration to the CONFIG class. An internal helper method _apply_config_dict() updates nested configuration attributes and top-level configuration settings, with proper error handling for missing files.

Changes

Cohort / File(s)	Summary
Changelog documentation CHANGELOG.md	Added release notes for version 4.1.3 documenting the re-addition of CONFIG.load_from_file() method
Configuration loading feature flixopt/config.py	Added public classmethod load_from_file(config_file) to load and parse YAML config files, and internal classmethod _apply_config_dict(config_dict) to apply parsed config to CONFIG class attributes with nested configuration support

Sequence Diagram

sequenceDiagram
    participant Caller
    participant CONFIG
    participant IO Module
    participant YAML Parser

    Caller->>CONFIG: load_from_file(config_file)
    CONFIG->>IO Module: read file
    IO Module-->>CONFIG: file content or FileNotFoundError
    CONFIG->>YAML Parser: parse YAML
    YAML Parser-->>CONFIG: config_dict
    CONFIG->>CONFIG: _apply_config_dict(config_dict)
    CONFIG->>CONFIG: update Modeling, Solving, Plotting, etc.
    CONFIG-->>Caller: CONFIG class (for chaining)

Loading

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

• Review focus areas:
  • Validation logic for file existence and YAML parsing correctness
  • Nested attribute update mechanism in _apply_config_dict() to ensure all configuration categories (Modeling, Solving, Plotting) are properly handled
  • Error handling for missing files and malformed YAML

Possibly related PRs

• flixOpt/flixopt#385: Directly related as it modifies the same CONFIG API by introducing load_from_file() and _apply_config_dict() methods with identical functionality

Poem

🐰 A config file lost, now returns with care,
Load from YAML, apply with flair,
Nested settings bloom, validation's there,
Chaining methods dance through the air! 📋✨

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/load-config-from-file

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between b10dfe8 and a1929ac.

📒 Files selected for processing (2)

• CHANGELOG.md (1 hunks)
• flixopt/config.py (1 hunks)

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