changed: consistency for return values in f̂!

Author: franckgaga

src/estimator/internal_model.jl

@@ -177,7 +177,7 @@ It calls [`f!`](@ref) directly since this estimator does not augment the states.
 function f̂!(x̂0next, _ , k0, estim::InternalModel, model::NonLinModel, x̂0, u0, d0)
     f!(x̂0next, k0, model, x̂0, u0, d0, model.p)
     x̂0next .+= estim.f̂op .- estim.x̂op
-    return x̂0next
+    return nothing
 end
 
 @doc raw"""