message: Implement DiffArray::copy()
diff: @@ -1504,6 +1504,13 @@ struct DiffArray : ArrayBase<value_t<Type_>, is_mask_v<Type_>, DiffArray<Type_>>
  //! @{ \name Miscellaneous
  // -----------------------------------------------------------------------
  + DiffArray copy() const {
  + if constexpr (is_jit_array_v<Type>) {
  + return create(m_index, m_value.copy());
  + } else
  + enoki_raise("copy(): not supported in scalar mode!");
  + }
  +
  auto vcall_() const {
  if constexpr (is_jit_array_v<Type>)
  return m_value.vcall_();
  