Add to_parquet and to_csv methods for Dataset and Station classes

[Copilot]

This PR implements new data export methods for the MetObs-toolkit, allowing users to save processed observations with QC flags to parquet and CSV formats.

Added Methods

Dataset Class

• to_parquet(target_file, **kwargs) - Export multi-station observations to parquet format
• to_csv(target_file, **kwargs) - Export multi-station observations to CSV format

Station Class

• to_parquet(target_file, **kwargs) - Export single station observations to parquet format
• to_csv(target_file, **kwargs) - Export single station observations to CSV format

Implementation Details

All methods follow the established codebase patterns:

• Use the @log_entry decorator for consistent logging
• Export the DataFrame from the .df property (includes processed data with QC labels)
• Accept **kwargs to pass additional options to the underlying pandas methods
• Include comprehensive docstrings with parameters, returns, and "See Also" sections

Usage Examples

# Export entire dataset with all stations
dataset.to_parquet('observations.parquet')
dataset.to_csv('observations.csv')

# Export individual station data
station = dataset.get_station('vlinder05')
station.to_parquet('vlinder05_data.parquet')
station.to_csv('vlinder05_data.csv')

# Use pandas options for customization
dataset.to_csv('data.csv', sep=';', float_format='%.2f')
station.to_parquet('data.parquet', compression='gzip')

Testing and Documentation

• Added comprehensive tests in test_importing.py following the existing save_dataset_to_pkl test pattern
• Updated API documentation for both Dataset and Station classes
• Added new "Exporting data" section to the introduction notebook with practical examples
• Applied black code formatting for style consistency

The implementation enables users to easily export their processed meteorological observations for further analysis or sharing while preserving all quality control information.

Fixes #553.

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[Copilot]

Pull Request Overview

This PR adds data export functionality to the MetObs-toolkit by implementing to_parquet and to_csv methods for both Dataset and Station classes. These methods enable users to export processed meteorological observations with quality control flags to standard data formats.

Key changes:

• Added export methods that leverage pandas DataFrame serialization capabilities
• Implemented comprehensive test coverage with round-trip validation
• Added pyarrow dependency for parquet support

Reviewed Changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_importing.py	Added four comprehensive test methods validating round-trip export/import for both formats and classes
src/metobs_toolkit/station.py	Implemented to_parquet and to_csv methods for Station class with proper logging and documentation
src/metobs_toolkit/dataset.py	Implemented to_parquet and to_csv methods for Dataset class with proper logging and documentation
pyproject.toml	Added pyarrow dependency for parquet format support
docs/reference/station.rst	Updated API documentation to include new export methods
docs/reference/dataset.rst	Updated API documentation to include new export methods