Add generate_changed_test_data method to simulate external disrupting events

- Added generate_changed_test_data() method to SmartSupplyChainDataGenerator
- Method takes disruption config with month_to_change and demand_ratio parameters
- Supports applying disruptions to specific watches or all watches
- Recalculates all financial metrics based on modified demand
- Preserves original demand values for comparison
- Generates data_test_external_events.json with disrupted test data
- Updated main() to demonstrate usage with example disruptions
- Added /data to .gitignore to exclude generated data files