Look for the closest airport when using plane

[tomasbastien]

Hello there !

As the lookup already exists for train station, it could be relevant to search for the closest airport of the user provided location (with OSM POI for example).

I've been working on a similar tool on my side, hightly motivated by volonteering for POW France (available on https://protectourwinters.fr/outil-co2e/ and code on https://github.com/tomasbastien/co2e-calculator), I'll be sharing common concerns that I encountered in separated issues in this repo