whenxuan: update the version for 0.0.9

[wwhenxuan]

This pull request updates the package version to 0.0.9 and introduces several improvements and corrections to the augmentation module. The most notable changes are the renaming of a key augmentation function, updates to the test suite to reflect these changes, and minor code cleanups.

Version and metadata updates:

• Updated the version number from 0.0.8 to 0.0.9 in both s2generator/__init__.py and setup.py. [1] [2]

Augmentation module corrections and improvements:

• Renamed the augmentation function and its imports from empirical_model_modulation to empirical_mode_modulation in s2generator/augmentation/__init__.py. [1] [2]
• Updated the test imports in tests/test_augmentation.py to use empirical_mode_modulation and added new augmentation functions to the test suite.
• Changed the argument name from series to time_series in the frequency_perturbation test to match the updated function signature.

Code cleanup:

• Minor formatting improvement in val_diff method in s2generator/base.py for readability and consistency.