Add a warning in ros2node when dealing with non-uniquely-named nodes

[emersonknapp]

Feature request

Related to #279 - but a much simpler feature request

• When running ros2 node list, add a top level warning and a small note next to all nodes that share a name, e.g.

$ ros2 node list
# WARNING: There are nodes in the graph that share a name - it is recommended that you use namespaces or different names to differentiate nodes
/talker (not uniquely named)
/listener
/talker (not uniquely named)

• When running ros2 node info, add a note that you are receiving information about only one node with the given name, if multiple are present, e.g.

$ ros2 node info /talker
# WARNING: there is more than one node found that has the name /talker, you are seeing information about only one of them. It is not guaranteed that you will see information about the same node every time
etc...

This should help make the non-unique situation more obvious, even though it doesn't fix anything.

Implementation considerations

We should already have all the information we need, a sort and pass over the node list should do for both cases.

[hidmic]

@emersonknapp as far as I can see, this issue has been fully addressed. I'll close it.