This commit introduces a new `ArithmeticDataset` class that:
- Combines complex and simple arithmetic task generation approaches
- Provides configurable task generation via `ArithmeticDatasetConfig`
- Supports deterministic task generation
- Implements dataset interface for easy use with HuggingFace datasets
- Adds comprehensive test coverage for the new implementation
Changes include:
- Refactored `basic_arithmetic.py` to use a unified dataset approach
- Added configuration validation and flexible generation options
- Created test suite to validate dataset behavior
- Removed file generation methods in favor of in-memory generation
