Documentation Improvement
- Added example usage for the `determine_periods` function, including default options and a CSV export feature.
- Included output examples with figures (`test_default.png`, `test_steps_default.png`) showcasing vorticity data with detected cyclone life cycle phases and step-by-step didactic plots.
- Added a section on customizing filtering parameters, including options for `cutoff_low`, `cutoff_high`, and smoothing settings (`use_filter`, `use_smoothing`, `use_smoothing_twice`).
- Introduced detailed explanations for key arguments like `series`, `x`, `plot`, `plot_steps`, and filtering options in the `usage.rst` file.
- Modified the layout of output examples in `usage.rst`, making it more user-friendly and emphasizing the CSV output, figures, and filtering customization.
- Added clarification in the "Important Notes" section explaining the processing of Northern Hemisphere (NH) data through multiplication of vorticity values by -1.