Enhance type annotations and refactor code for improved clarity and maintainability

- Added type hints across various modules to improve type checking and code readability.
- Introduced `PlDataProtocol` and `PlModelProtocol` to define structural protocols for data and model containers, enhancing type safety.
- Refactored several functions to specify input and output types, reducing reliance on `Any`.
- Updated the `plot_data_prep.py` and `model_math.py` modules to utilize new type hints and protocols.
- Adjusted import statements to include necessary types and maintain compatibility with mypy.
- Fixed minor issues in plotting functions and data handling to ensure consistency with new type definitions.
- Updated `pyproject.toml` to include mypy configuration for stricter type checking.
- Modified test paths in `test_exptime_integration.py` to reflect new directory structure.
- Fix Ruff linting and mypy type errors across test files