The vector is filled with pointers to base nodes.

The existing clone function is a direct copy, but
setting different nodes is required.

Since boost-1.61 (and also not in the std::optional)
optionals holding references do have some restrictions
and are difficult to use. In this special case
optional<PropertyVector&> is equivalent semantically
to a PropertyVector*. The pointer indicates that it is
not owning (contrary to usage of unique_ptr).

Thanks to Christoph pointing this issue out.

Allow modification of the component of the specified tuple.

The called getPropertyVector<T>() did not return
a const reference optional, but an optional<T&>.

With boost 1.61 this conversion is no longer possible.

Fixes #1383