To sum up:
 - because the Mesh object and the node pointers are living longer than a mesh subset, it is safe to copy pointers to Nodes.
 - it is probably *not* safe to continue using the `vec_items` pointer after construction of a `MeshSubset`. On the other hand copying nodes one more time is (memory) expensive.

I'd keep it like it is now, to see its real usage in near feature.