improve model display in docs

Author: TorkelE

docs/src/model_creation/functional_parameters.md

@@ -56,7 +56,7 @@ in_type = typeof(interpolated_light)
 @parameters kA kD (light_in::in_type)(..)
 @species Pₐ(t) Pᵢ(t)
 rxs = [
-    Reaction(kA*light_in(t), [Pᵢ], [Pₐ]),
+    Reaction(light_in(t)*kA, [Pᵢ], [Pₐ]),
     Reaction(kD, [Pₐ], [Pᵢ])
 ]
 @named rs = ReactionSystem(rxs, t)
@@ -77,7 +77,7 @@ It is possible to use time-dependent inputs when creating models [through the DS
 ```@example functional_parameters_circ_rhythm
 input = light_in(t)
 rs_dsl = @reaction_network rs begin
-    (kA*$input, kD), Pᵢ <--> Pₐ
+    ($input*kA, kD), Pᵢ <--> Pₐ
 end
 ```
 We can confirm that this model is identical to our programmatic one (and should we wish to, we can simulate it using identical syntax syntax).
@@ -115,9 +115,10 @@ using Catalyst
 @species I(default_t())
 inf_rate = I_rate(I)
 sir = @reaction_network rs begin
-    k1*$(inf_rate), S --> I
+    k1*$inf_rate, S --> I
     k2, I --> R
 end
+nothing # hide
 ```
 Finally, we can simualte our model.
 ```@example functional_parameters_sir

src/reactionsystem.jl

@@ -354,11 +354,11 @@ struct ReactionSystem{V <: NetworkProperties} <:
         end
 
         if isempty(sivs) && (checks == true || (checks & MT.CheckUnits) > 0)
-            #if !all(u == 1.0 for u in ModelingToolkit.get_unit([unknowns; ps; iv]))
-            #    for eq in eqs
-            #        (eq isa Equation) && check_units(eq)
-            #    end
-            #end
+            if !all(unitless_symvar(sym) for sym in [unknowns; ps; iv])
+                for eq in eqs
+                    (eq isa Equation) && check_units(eq)
+                end
+            end
         end
 
         rs = new{typeof(nps)}(
@@ -1549,6 +1549,6 @@ unitless_exp(u) = iscall(u) && (operation(u) == ^) && (arguments(u)[1] == 1)
 
 # Checks if a symbolic variable is unitless. Also accounts for callable parameters (which
 # should not have units but for which `get_unit` is undefined: https://github.com/SciML/ModelingToolkit.jl/issues/3420).
-function unitless_symvar(sym) 
+function unitless_symvar(sym)
     return (sym isa Symbolics.CallWithMetadata) || (ModelingToolkit.get_unit(sym) == 1)
 end