replace copy with similar for performance

Author: vpuri3

src/func.jl

@@ -265,13 +265,13 @@ Base.:\(L::FunctionOperator{false}, u::AbstractVecOrMat) = L.op_inverse(u, L.p,
 
 function Base.:*(L::FunctionOperator{true}, u::AbstractVecOrMat)
     _, co = L.cache
-    du = copy(co)
+    du = similar(co)
     L.op(du, u, L.p, L.t)
 end
 
 function Base.:\(L::FunctionOperator{true}, u::AbstractVecOrMat)
     ci, _ = L.cache
-    du = copy(ci)
+    du = similar(ci)
     L.op_inverse(du, u, L.p, L.t)
 end