Model constructors with positional SpectralGrid argument

[milankl]

At the moment one has to write

model = PrimitiveWetModel(; spectral_grid, kwargs...)

to construct any model. To lower the confusion for beginners ("what's that ; for?") also allow for the spectral grid to be passed on as first positional argument, i.e.

model = PrimitiveWetModel(spectral_grid, kwargs...)

which is also consistent with how we create all model components.

[maximilian-gelbrecht]

Yeah, I am fine with that, as it's a required argument, this could avoid confusion.