use tuple isntead of mixed array

Author: TorkelE

test/miscellaneous_tests/symbolic_stoichiometry.jl

@@ -58,7 +58,7 @@ begin
     u0_1 = (A => 3.0, B => 2.0, C => 3.0, D => 5.0)
     ps_1 = (k => 5.0, α => 2)
     u0_2 = [u[2] for u in u0_1]
-    ps_2 = [p[2] for p in ps_1]
+    ps_2 = Tuple(p[2] for p in ps_1)
     τ = 1.5
 
     # Creates `ReactionSystem` model.