feat: only consider cities from the most likely country

[MartinKolarik]

Sometimes, we run into a case where we get multiple cities that are different but close to each other, and then one somewhere far, reported by two providers, e.g.:

lisbon, PT
san francisco, US
san jose, US
new york, US
lisbon, PT

johannesburg, ZA
amsterdam, NL
lelystad, NL (almost amsterdam)
amsterdam, NL
johannesburg, ZA

Sometimes each provider returns a different city, but multiple agree on the country:

calais, FR
Paris, FR
brugge, BE
roubaix, FR

We currently don't consider at all how close or far the cities are, and doing so makes sense and does improve the results a little. I initially considered clustering the cities by distance based on coordinates, but this is easier and seems to work just as well.