New weather provider: Yr.no (#2948)

# Added Yr.no as a weather provider

Yr.no is a free Norwegian weather service. The configuration is quite
simple:
```js
{
    weatherProvider: "yr",
    lat: 59.9171,
    lon: 10.7276,
    altitude: 30
}
```
The latitude and longitude cannot have more than 4 decimals, but that
should be plenty. To quote yr: "There is no need to ask for weather
forecasts with nanometer precision!". The altitude should be meters
above sea level and defaults to 0. If `type` is set to `current` the
symbol can display the next 1, 6 or 12 hours by setting
`currentForecastHours` (default is 1).

It states in [Getting
started-guide](https://developer.yr.no/doc/GettingStarted/) that users
of the API should cache the results and use the `Expires`-header to know
when to ask for new data. By using the `If-Modified-Since`-header we can
avoid downloading the same data over and over again. I chose not to
override the `User-Agent`-header set in
[`server.js`](https://github.com/MichMich/MagicMirror/blob/a328ce5/js/server.js#L97)
even though it does not comply with [the terms of
service](https://developer.yr.no/doc/TermsOfService/). It currently
works with the default header, and by searching the web for MagicMirror
the GitHub-repo should be easy to find without an explicit link.

I also had to make some minor changes to `server.js` and
`weatherprovider.js` to be able to send and return HTTP headers. To
handle the HTTP 304 response without body I chose to return `undefined`
so we easily can use the response as a condition: `if (response) ...`.

The documentation for the API is available here:
- [API Reference overview](https://api.met.no/weatherapi/)
-
[Locationforecast](https://api.met.no/weatherapi/locationforecast/2.0/)
- Used to get the weather forecast
- [Sunrise](https://api.met.no/weatherapi/sunrise/2.0/documentation) -
used to find sunrise and sunset times

Co-authored-by: Veeck <github@veeck.de>

modules/default/weather/weather.js

@@ -58,7 +58,7 @@ Module.register("weather", {
 
 	// Return the scripts that are necessary for the weather module.
 	getScripts: function () {
-		return ["moment.js", "weatherutils.js", "weatherprovider.js", "weatherobject.js", "suncalc.js", this.file("providers/" + this.config.weatherProvider.toLowerCase() + ".js")];
+		return ["moment.js", this.file("../utils.js"), "weatherutils.js", "weatherprovider.js", "weatherobject.js", "suncalc.js", this.file("providers/" + this.config.weatherProvider.toLowerCase() + ".js")];
 	},
 
 	// Override getHeader method.