Use metric units internally in all weatherproviders (#2849)

So finally I think this refactorin is ready to be reviewed :-)

DONE:
- [x] Removed all conversion functions for wind and temperature from
specific weatherproviders
- [x] Use internally only metric units: celsius for temperature, meters
per seconds for wind
- [x] Convert temp and wind into the configured units when displaying
data on the UI
- [x] look how beaufort calculation uses metrics, added knots as new
windunit
- [x] add more e2e tests 

Checked providers:
- [x] Darksky
- [x] EnvCanada
- [x] OpenWeatherMap
- [x] SMHI provider 
- [x] UK Met Office
- [x] UK Met Office DataHub
- [x] WeatherBit
- [x] WeatherFlow
- [x] WeatherGov

TODO in different tickets:
- check weatherproviders for usage of weatherEndpoint (as seen in
https://github.com/MichMich/MagicMirror-Documentation/issues/131) -> see
#2926
- cleanup precipations -> #2953

Co-authored-by: veeck <michael@veeck.de>

tests/electron/helpers/weather-setup.js

@@ -0,0 +1,29 @@
+const helpers = require("./global-setup");
+const path = require("path");
+const fs = require("fs");
+const { generateWeather, generateWeatherForecast } = require("../../mocks/weather_test");
+
+exports.getText = async (element, result) => {
+	const elem = await helpers.getElement(element);
+	await expect(elem).not.toBe(null);
+	const text = await elem.textContent();
+	await expect(
+		text
+			.trim()
+			.replace(/(\r\n|\n|\r)/gm, "")
+			.replace(/[ ]+/g, " ")
+	).toBe(result);
+};
+
+exports.startApp = async (configFile, systemDate) => {
+	let mockWeather;
+	if (configFile.includes("forecast")) {
+		mockWeather = generateWeatherForecast();
+	} else {
+		mockWeather = generateWeather();
+	}
+	let content = fs.readFileSync(path.resolve(__dirname + "../../../../" + configFile)).toString();
+	content = content.replace("#####WEATHERDATA#####", mockWeather);
+	fs.writeFileSync(path.resolve(__dirname + "../../../../config/config.js"), content);
+	await helpers.startApplication("", systemDate);
+};