Add runoff to Basin

[Huite]

The current Basin definition:

struct Basin{T, C} <: AbstractParameterNode
    node_id::Indices{NodeID}
    precipitation::Vector{Float64}
    potential_evaporation::Vector{Float64}
    drainage::Vector{Float64}
    infiltration::Vector{Float64}
    # cache this to avoid recomputation
    current_level::T
    current_area::T
    ...

does not have a vector for the urban runoff values.

Secondly, should it maybe just be runoff? Not clear whether we need the distinction.

For the coupling, MetaSWAP will just return runoff. Urban areas are separated in terms of their SVAT unit, but it's the same water balance term.

[gijsber]

distinction of urban runoff would be a good nice-to-have as it allows future distinction at the WWTP between dry-weather and wet-weather waste loads. If it becomes hard to make this seperation between urban and non-urban runoff, we should do it now, else we can postpone.

[visr]

To resolve this, we will rename urban_runoff to runoff and implement this similar to the way we implement drainage. Also needs to be integrated and added to basin.arrow like the other Basin vertical fluxes. Also needs doc updates.