Issue warning instead of error when independent variable is not a parameter

Author: hersle

src/utils.jl

@@ -116,7 +116,7 @@ const CheckUnits = 1 << 2
 function check_independent_variables(ivs)
     for iv in ivs
         isparameter(iv) ||
-            throw(ArgumentError("Independent variable $iv is not a parameter."))
+            @warn "Independent variable $iv should be defined with @independent_variables $iv."
     end
 end
 

test/odesystem.jl

@@ -1202,7 +1202,7 @@ end
     @test_nowarn @named sys = ODESystem([y ~ 0], x)
 
     @variables x y(x)
-    @test_throws ArgumentError @named sys = ODESystem([y ~ 0], x)
+    @test_logs (:warn,) @named sys = ODESystem([y ~ 0], x)
 
     @parameters T
     D = Differential(T)