forecast.el: darksky.net API to be closed in 2021; rewrite using another data provider

[cadadr]

As per #47 explains, DarkSky.net has been acquired by Apple, who plans to absorb the company and close up the APIs "by the end of 2021" (relevant announcement).

This means forecast.el needs to be rewritten using a new, preferably more open data provider. This is also an opportunity to detach forecast.el frontend and backend so that backends can be switched if new ones are implemented later. Maybe use EIEIO?

Candidates for the new backend:

• OpenWeatherMap (https://openweathermap.org/)
• MAAS (http://ingenology.github.io/mars_weather_api/) (why not, as an option)
• Wttr.in? I think they have a JSON api

Some additional ideas:

• Use geoclue.el for setting lat&long? (see this)

Alternatives for forecast.el (Apple will apply a new privacy policy):

• sunshine.el: uses OpenWeatherMap, less detailed but more beautiful than forecast.el
• noaa.el: uses data from US Govt's National Atmospheric and Oceanic Administration, which is seemingly limited to the USA
• weather-metno.el: Uses data from Norwegian Meteorologisk institutt which provides data for all world under a Creative Commons licence (e.g. see Istanbul)
• emacs-wttrin: A simple wrapper around https://wttr.in which also supports JSON output (thus potential backend, see this)
• yandex-weather.el: Gets data from Yandex.Weather

[cadadr]

re: forecast.el

An update from DarkSky.net I just discovered:

"Update: Support for the Dark Sky API service for existing customers will continue until the end of 2022. The iOS app and Dark Sky website will also be available until the end of 2022." (Published on June 7, 2021 at https://blog.darksky.net/, accessed February 2, 2022).

So if you had an existing API key, you can continue to use it with forecast.el this year, but you can't sign up for a new API key.

I was planning to stop developing forecast.el tbf, but I guess with this extension I may re-write it in summer to use a new API, and decouple things so multiple data backends can be used.

[cadadr]

A couple days ago I received an email from Apple that said they'd close the API in March 2023 and replace it with a "Weather Kit".

I don't know if that WeatherKit is usable as a general purpose API, but the message did say it would be "for web".

[cadadr]

cf. 7ae7864 and #61