Nycflights13

* Fixes for R CMD check changes.

* `airports$tz` with values `\N` replaced with `NA` (36).

* `weather$year` and `weather$month` are now integers to match `flights$year`
and `flights$month` (jozefhajnala, 34)

* `weather$time_hour` and `flights$time_hour` are now stored in the
America/New_York timezone (19).

* `weather` data updated from latest Iowa State mesonet (rmcd1024, 24).
`wind_gust` is now correctly captured from the underlying data, rather than
being a copy of `wind_speed`. `precip` is better captures the hourly
preciptation (which tends to be recorded at 51 minutes past the hour) (27)

* Import function from tibble to suppress R CMD check NOTE.

* nycflights imports tibble so you get nice printing even when no other
tidyverse package is loaded.

* `airports` now has a `tzone` column that contains the IANA time zone
for the airport (15).

* `airlines`: `carrier` columns are characters instead of factors (2).

* `airports`: duplicate entry for BFT removed (7).

* `flights`:

* new `time_hour` variable combines `year`, `month`, `day`, and
`hour` into a single variable (11).

* new `sched_dep_time` and `sched_arr_time` variables give scheduled
departure and arrival times - these are more appropriate for connecting
to weather data. `hour` and `minute` are now computed from the scheduled
departure time, not the actual departure time.

* missing `tailnum` now recorded as `NA`, not `""` (10).

* `weather`:

* Includes weather data for all airports.

* New `time_hour` variable combines `year`, `month`, `day`, `hour` into
a single POSIXct variable.

* Saved as ungrouped.