Add `trim` function to parameter `FHRU` of HydPy-L

[tyralla]

Currently, users can assign relative areas to parameter FHRU that do not sum up to one. Potential errors could be easily avoided by implementing a trim function similar to the one of parameter ZoneArea (similar but not equal, as ZoneArea's unit is km²).

When we do so, we should also check the corresponding parameters of other model families (not only regarding areas but also lengths) and eventually extract the common functionality into a shared base class.