whenxuan: update the version for 0.0.9

[wwhenxuan]

This pull request updates the S2Generator package to version 0.0.9 and includes several improvements and fixes to the augmentation module and its tests. The main changes involve correcting naming inconsistencies, updating function signatures and return values, and ensuring that the tests align with these updates.

Augmentation module improvements:

• Fixed a naming inconsistency in the augmentation module: renamed all references from empirical_model_modulation to empirical_mode_modulation in both the import statements and the __all__ list. [1] [2]
• Updated the amplitude_modulation function to return only the modulated series, removing the second return value.

Testing updates:

• Updated test_amplitude_modulation to use the new amplitude_mean and amplitude_variation parameters instead of min_modulation and max_modulation, and adjusted the function call accordingly.
• Updated test_frequency_perturbation to use the correct time_series parameter name and argument order in the function call.
• Added missing imports for new augmentations (empirical_mode_modulation, wiener_filter, add_linear_trend, time_series_mixup) in the test suite.

Versioning:

• Bumped the package version to 0.0.9 in both setup.py and s2generator/__init__.py. [1] [2]

Minor code cleanup:

• Minor formatting and code clarity improvements in val_diff for multivariate input handling.