feat: add specialized observed for AbstractDDEProblem

Author: AayushSabharwal

src/problems/dde_problems.jl

@@ -259,6 +259,14 @@ function DDEProblem(f::AbstractDDEFunction, args...; kwargs...)
     DDEProblem{isinplace(f)}(f, args...; kwargs...)
 end
 
+function SymbolicIndexingInterface.observed(prob::AbstractDDEProblem, sym)
+    return let hist = prob.h,
+        rawfn = SymbolicIndexingInterface.observed(symbolic_container(prob), sym)
+
+        fn(u, p, t) = rawfn(u, hist, p, t)
+    end
+end
+
 """
 $(TYPEDEF)
 """