Interface for frequency and time dependent models

[ptiede]

Given that #29 has landed, we can add better multifrequency support for models. We will not take a wrapper approach since unwrapping every multifrequency model would be very difficult. Instead, the current plan is to tag the parameters passed to the model themselves.

A spectral index model could look something like

struct SpectralIndex{T, F}
      specindx::T
      ref_freq::F
end

function build_params(model::TaylorSpectral, refparam, Ti, Fr)
       param = refparam*(Fr/model.ref_freq)^model.specindex
       return param
end

struct MultiDomainParam{Fr, Ti, P}
      freqmodel::Fr
      timemodel::Ti
      param::P
end

modify(Gaussian(), Stretch(MultiDomainParam(2.0, SpectralIndex(1.0, 230e9), Constant()))