Some cities (e.g. in different countries) have the same names. Providing a city
name to the weather module would previously display the first returned result by openweathermap's API.
This commit allows city ids to be provided to the weather module. If a location passed
to the weather module only contains numbers, it will be interpreted as a city id.
City ids are those handled by openweathermap's API to uniquely identify cities.
See https://openweathermap.org/current#cityid for details.
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