fix(bus): Add plan_stop_departure_dt to headway calculations

[runkelcorey]

Asana Task: 🚌 Add plan_stop_departure_dt to ordering of headways

What changes does this PR propose?

1. Makes all datetimes UTC (so they are combinable)
2. Adds plan_stop_departure_dt to the calculation of headways

How were these changes validated?

1. Wrote a new test dataframely class with a rule that any row with non-empty inputs should have non-empty headways

What questions should reviewers consider?

None.

[github-actions]

LCOV of commit 46e2b6d during Continuous Integration (Python) #1998

Summary coverage rate:
  lines......: 64.7% (3598 of 5561 lines)
  functions..: 29.9% (275 of 920 functions)
  branches...: no data found

Files changed coverage rate:
                                                                                     |Lines       |Functions  |Branches    
  Filename                                                                           |Rate     Num|Rate    Num|Rate     Num
  =========================================================================================================================
  src/lamp_py/bus_performance_manager/combined_bus_schedule.py                       |97.2%     36|50.0%     2|    -      0
  src/lamp_py/bus_performance_manager/events_gtfs_schedule.py                        | 100%     70|50.0%    10|    -      0
  src/lamp_py/bus_performance_manager/events_metrics.py                              |67.2%     58|33.3%    12|    -      0
  src/lamp_py/bus_performance_manager/events_tm_schedule.py                          | 100%     34|50.0%     2|    -      0
  src/lamp_py/tableau/conversions/convert_bus_performance_data.py                    |34.8%     23|25.0%     4|    -      0

[huangh]

Did you confirm that all these date times that were tz-less before were all tz-less UTC, and we haven't assigned UTC to EST times?

I think there's a test in there somewhere that expects the correct relative difference between time fields, but we should ensure that nothing has changed here.

[runkelcorey]

Did you confirm that all these date times that were tz-less before were all tz-less UTC, and we haven't assigned UTC to EST times?

yep!