refactor: use @structural_parameters to pass functions to StaticNonLinearity, TimeVaryingFunction

Author: ven-k

src/Blocks/math.jl

@@ -221,15 +221,15 @@ If the given function is not composed of simple core methods (e.g. sin, abs, ...
   - `output`
 """
 @mtkmodel StaticNonLinearity begin
-    @parameters begin
+    @structural_parameters begin
         func
     end
     @extend u, y = siso = SISO()
     @equations begin
-        y ~ first(getdefault(func))(u)
+        y ~ func(u)
     end
 end
-StaticNonLinearity.f(func; name) = StaticNonLinearity.f(; name = name, func = [func])
+StaticNonLinearity.f(func; name) = StaticNonLinearity(; func, name)
 
 """
     Abs(; name)

src/Blocks/sources.jl

@@ -97,17 +97,17 @@ The input variable `t` can be changed by passing a different variable as the key
 - `output`
 """
 @mtkmodel TimeVaryingFunction begin
-    @parameters begin
+    @structural_parameters begin
         f
     end
     @components begin
         output = RealOutput()
     end
     @equations begin
-        output.u ~ first(getdefault(f))(t)
+        output.u ~ f(t)
     end
 end
-TimeVaryingFunction.f(f; name) = TimeVaryingFunction.f(; f = [f], name)
+TimeVaryingFunction.f(f; name) = TimeVaryingFunction(; f, name)
 
 """
     Sine(; name, frequency, amplitude = 1, phase = 0, offset = 0, start_time = 0,