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/modules/weather_spec.js

@@ -0,0 +1,28 @@
+const helpers = require("../helpers/global-setup");
+const weatherHelper = require("../helpers/weather-setup");
+
+describe("Weather module", () => {
+	afterEach(async () => {
+		await helpers.stopApplication();
+	});
+
+	describe("Current weather with sunrise", () => {
+		beforeAll(async () => {
+			await weatherHelper.startApp("tests/configs/modules/weather/currentweather_default.js", "13 Jan 2019 00:30:00 GMT");
+		});
+
+		it("should render sunrise", async () => {
+			await weatherHelper.getText(".weather .normal.medium span:nth-child(4)", "7:00 am");
+		});
+	});
+
+	describe("Current weather with sunset", () => {
+		beforeAll(async () => {
+			await weatherHelper.startApp("tests/configs/modules/weather/currentweather_default.js", "13 Jan 2019 12:30:00 GMT");
+		});
+
+		it("should render sunset", async () => {
+			await weatherHelper.getText(".weather .normal.medium span:nth-child(4)", "3:45 pm");
+		});
+	});
+});