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/e2e/modules/weather_forecast_spec.js

@@ -86,7 +86,7 @@ describe("Weather module: Weather Forecast", () => {
 			await weatherFunc.startApp("tests/configs/modules/weather/forecastweather_units.js", {});
 		});
 
-		const temperatures = ["24_4°", "21_0°", "22_9°", "23_4°", "20_6°"];
+		const temperatures = ["75_9°", "69_8°", "73_2°", "74_1°", "69_1°"];
 		for (const [index, temp] of temperatures.entries()) {
 			it("should render custom decimalSymbol = '_' for temp " + temp, async () => {
 				await weatherFunc.getText(`.weather table.small tr:nth-child(${index + 1}) td:nth-child(3)`, temp);