This function takes two lists of airports (of the same length), specified
as 4-letter codes and combines them, adding the fields:
from_long, from_lat, to_long, to_lat
: the airport lat-longs
with adep first
AP2
: a name for the route in a specific order
gcdist_km
: the great circle distance in km
In AP2
European airports (crudely, from starting letter = 'E' or 'L')
are listed first, otherwise in alphabetical order. If unidirectional is TRUE,
then ">" is the separator, otherwise "<>".
(Unidirectional not currently supported)
For more details see the introductory vignette.