Refactor nearest neighbor and interpolation steps

[tiffanychu90]

segment speeds refactor

• Follow-up to Remove vp nearest neighbor intermediate output #1319 and add the dummy arrival time needed to get cut_stop_segments.py to run successfully
• Refactor nearest neighbor and interpolation steps
  • Previously, nearest neighbor was broken out into 2 intermediate outputs, nearest 10 vp then filtering to nearest 2 vp
  • Now, we can work all that into the same numpy array and get exactly what we want from the nearest neighbor step
  • Rather than reshaping within interpolation step, just get exactly the wide shape we want when we save out the nearest neighbor output in stage2
  • Clean up the extra entries in gtfs_analytics_data.yml and remove the need for stage2b...we can just use stage2 again
  • 2 nearest neighbor steps took roughly ~30 min, and now swapping it out into 1 step, the times stay the same, but opting for simplicity
  • interpolation step took ~15 min, and now it's 9 min
  • Remove extra functions no longer needed
• Refactor functions in segment_speed_utils.neighbor
• Remove test scripts after testing 1 date
• Add the 2 speed layer URLs to open_data/README.md
• Epic - GTFS analytics pipeline performance improvements #1315