Pipeline improvements

[kaushik12]

• Add checks in the pipeline to see if raw/processed data exists to avoid re-fetching existing data
• Add a force/clean parameter in the pipeline to bypass checks and run the entire process from scratch
• Modify parsing of city parameter in run_pipeline to accept multiple cities

The improvements are to make it easier to add multiple cities at once to the config and re-run the pipeline optimally without having to regenerate existing data.