Refactor `WeatherSource` and `WeatherSourceWrapper`

[sebastian-peter]

WeatherSource:

• WeatherSource#apply and WeatherSource#checkConfig can be consolidated. checkConfig is a bad name considering what the method is doing, anyway.
• Creation of WeatherSourceWrapper in line 344ff can be simplified

WeatherSourceWrapper:

• The usage of implicits in all WeatherSourceWrapper#apply methods is unnecessary and confusing

There's probably more once one looks deeper into this