Unique index nodes

Hi, this is my attempt to implement the creation of unique nodes with an index. See #66

I wrote a test for my command and I am not so sure how to test the rest. I noticed that not everything is well unit tested.

The reason to make a new command and not to extend the old simply is that: it is possible to have a node without a id, the old command always checked for an id and did not store this id in the newly created node.