* Convert to use WeatherFormula::Units provided by the ions, currently only ported ion is ion_willab.
-- This allows us to convert between different units more easily as there's no need to guess which kind of unit format the ion is using.

* Introduced getUnitString() to get a string for WeatherFormula::Units, this could be moved to WeatherFormula namespace and let the applet developers to parse that data as wanted.
-- This is used to decide which frames to show from the SVG file.

Now it is possible for user to select wanted unit from the config and it will be automatically converted to the same format.