EDT-778: Allow editing the child element of cloned cluster if the original child has already been specialised. Previously, all child elements of the cloned cluster were copied as references. This had the unfortunate consequence that, if the original element was already at the same specialisation level as the archetype, the reference to the element could not be further specialised and therefore was not editable. This is fixed by cloning any child element that is already at the same specialisation level as the archetype; the cloned child is specialised and therefore ready for editing.