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>

modules/default/weather/current.njk

@@ -3,15 +3,7 @@
         <div class="normal medium">
             <span class="wi wi-strong-wind dimmed"></span>
             <span>
-                {% if config.useBeaufort %}
-                    {{ current.beaufortWindSpeed() | round }}
-                {% else %}
-                    {% if config.useKmh %}
-                        {{ current.kmhWindSpeed() | round }}
-                    {% else %}
-                        {{ current.windSpeed | round }}
-                    {% endif %}
-                {% endif %}
+                {{ current.windSpeed | unit("wind") | round }}
                 {% if config.showWindDirection %}
                     <sup>
                         {% if config.showWindDirectionAsArrow %}