fix: correct timestamp calculations in flight datasets & add generation script (#626)

* fixes dates of edge cases of flights-2k in which delay pushes date across midnight

* fixes dates of edge cases where delays shift dates for flights 5k, 10k, 20k json

* updates flights-3m.csv with correct number of rows

* adds flight-generation script

* updates flights.py docs

* refines docs for flights.py

Author: dsmedia