Gtfs funnel performance improvements

[tiffanychu90]

gtfs_funnel

• do a pass at refactoring several scripts in gtfs_funnel, esp the ones touching large dfs like stop_times and vp
• refactor stop_times_direction.py, vp_dwell_time.py, vp_condenser.py
  • simplify functions where possible
  • rewrite to use arrays and vectorized operations. rather than passing dfs, opt for using lists or series to get the subsets that require more preprocessing.
  • make better use of parquet filtering to grab only what's necessary
• consolidate vp_keep_usable.py and vp_direction.py into 1 script
  • remove vp_direction, keep vp_keep_usable only
• Update README and mermaid.md diagrams to reflect the fewer steps
• Epic - GTFS analytics pipeline performance improvements #1315