Add date filter functionality to get_gtfs_segments and get_bus_feed

[praneethd7]

Summary

This PR enhances the date filter functionality in get_gtfs_segments and get_bus_feed functions, adding support for single and multiple date inputs with comprehensive validation and error handling.

Key Changes

• Enhanced date filtering: Added support for analyzing specific dates instead of just the busiest day
• Flexible input formats: Supports both string ("YYYYMMDD") and datetime.date objects
• Multiple date analysis: Can analyze multiple dates simultaneously
• Input validation: Comprehensive error handling for invalid date formats and values
• Updated documentation: Added examples and usage instructions for the new date filtering features
• Test coverage: Added comprehensive tests for the new functionality

Files Modified

• gtfs_segments/gtfs_segments.py - Core functionality enhancements
• gtfs_segments/partridge_func.py - Supporting functions for date handling
• gtfs_segments/__init__.py - API updates
• README.md - Documentation updates with usage examples
• docs/usage.md - Detailed usage documentation
• test/test_gtfs_segments.py - New test cases
• requirements.txt - Updated dependencies

Usage Examples

# Single date analysis
segments_df = get_gtfs_segments("gtfs.zip", date="20240317")

# Multiple dates analysis  
segments_df = get_gtfs_segments("gtfs.zip", date=["20220315", "20220316", "20220317"])

# Mixed date formats
from datetime import date
segments_df = get_gtfs_segments("gtfs.zip", date=["20220315", date(2022, 3, 16)])

Testing

• All existing tests continue to pass
• New comprehensive test suite for date filtering functionality
• Covers edge cases and error conditions

Breaking Changes

None - this is a backward-compatible enhancement. Default behavior (analyzing busiest day) remains unchanged.