docs: remove old defining new models section

docs/src/models/models.md

@@ -15,45 +15,6 @@ PowerLaw
 BlackBody
 ```
 
-### Defining new models
-
-To define a new Julia model, we need only implement the [`AbstractSpectralModel`](@ref) interface. As a pedagogical example, consider an implementation of [`PowerLaw`](@ref):
-
-```julia
-# define the model and parameters
-@with_kw struct PowerLaw{F1,F2} <: AbstractSpectralModel
-    "Normalisation."
-    K::F1 = FitParam(1.0)
-    "Photon index."
-    a::F2 = FitParam(0.5)
-end
-
-# tell the package what kind of model this is
-modelkind(::Type{<:PowerLaw}) = Additive()
-
-# define how the model acts: 
-# all of the parameters are passed, in order, as arguments
-function SpectralFitting.invoke!(flux, energy, ::Type{<:PowerLaw}, a)
-    α = 1 - a
-    α⁻¹ = inv(α)
-    SpectralFitting.finite_diff_kernel!(flux, energy) do E
-        α⁻¹ * E^α
-    end
-end
-```
-
-Note that each parameter has its own parametric type, here `::F1` and `::F2`. This is so that the type of fit parameters can be changed, or alternative number types inserted (see [Why & how](@ref)). The standard parameter type is [`FitParam`](@ref), which is what we have used here.
-
-The model is defined to be additive by overloading [`modelkind`](@ref).
-
-!!! danger
-    Additive models _must have_ a normalisation parameter with the symbol `K`, which is, however, _not passed_ to [`SpectralFitting.invoke!`](@ref). Multiplicative and convolutional models have no such requirements.
-
-We have also used the [`SpectralFitting.finite_diff_kernel!`](@ref) utility function, which is designed to help us efficiently compute finite-difference integrations of flux, just as XSPEC does.
-
-!!! note
-    To add additional XSPEC models, see [Wrapping new XSPEC models](@ref).
-
 ## XSPEC models
 
 XSPEC models frequently have tabular data dependencies, without which the models fail to invoke (see [Model data availability](@ref)). If the data files are known but not present, the XSPEC models will throw an error with instructions for downloading the missing data. If the data files are unknown, Julia may crash catastrophically. If this is the case, often a single line will be printed with the LibXSPEC error, specifying the name of the missing source file. This can be registered as a data dependency of a model using [`SpectralFitting.register_model_data`](@ref).

docs/src/models/using-models.md

@@ -97,6 +97,9 @@ domain = collect(range(0.1, 10.0, 100))
 invokemodel(domain, model)
 ```
 
+!!! note
+    To add new XSPEC or foreign function models, see [Wrapping new XSPEC models](@ref).
+
 ## Model abstraction
 
 All spectral models are a sub-type of [`AbstractSpectralModel`](@ref).