Returns the departure at the lowest `stop_sequence` for every trip.
References
[GTFS Schedule Reference](https://gtfs.org/documentation/schedule/reference/#stop_timestxt)
Examples
head(get_1stdeparture(for_rail_gtfs))
#> # A tibble: 6 × 4
#> route_id trip_id departure_time stop_id
#> <chr> <chr> <chr> <chr>
#> 1 7 4 05:30:00 29
#> 2 7 5 06:15:00 29
#> 3 7 6 07:00:00 29
#> 4 7 7 07:45:00 29
#> 5 7 8 08:30:00 29
#> 6 7 9 10:15:00 29