fix: propagate is_dde through flatten and compose

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -2960,6 +2960,9 @@ function compose(sys::AbstractSystem, systems::AbstractArray; name = nameof(sys)
     nsys == 0 && return sys
     @set! sys.name = name
     @set! sys.systems = [get_systems(sys); systems]
+    if has_is_dde(sys)
+        @set! sys.is_dde = _check_if_dde(equations(sys), get_iv(sys), get_systems(sys))
+    end
     return sys
 end
 function compose(syss...; name = nameof(first(syss)))

src/systems/diffeqs/odesystem.jl

@@ -382,6 +382,7 @@ function flatten(sys::ODESystem, noeqs = false)
             defaults = defaults(sys),
             name = nameof(sys),
             initialization_eqs = initialization_equations(sys),
+            is_dde = is_dde(sys),
             checks = false)
     end
 end