Enable interpolation of functional variable (but not its argument).

[TorkelE]

Especially relevant after #1191 gets merged.

Currently, it is not possible to interpolate only a functional parameter into the DSL. Here is the current workflow:

using Catalyst

# Defines an input process (modified sinus wave).
tend = 5.0
ts = collect(0.0:0.0001:tend)
A = 2.0; f = 1.6; ϕ = 0.5;
spline = LinearInterpolation(A .* (sin.(2π * f .* ts .- ϕ) .+ 1) /2, ts)
@parameters (pIn::typeof(spline))(..)

# Defines a `ReactionSystem` with the input parameter (using the DSL).
input = pIn(t)
rs_pIn_dsl = @reaction_network rs_pIn begin
    ($input,d), 0 <--> X
    (k1*X,k2), Y1 <--> Y2
end

Ideally, one shoudl be able to do this:

rs_pIn_dsl = @reaction_network rs_pIn begin
    (($pIn)(t),d), 0 <--> X
    (k1*X,k2), Y1 <--> Y2
end