This represents an unique identifier of a node within an individual
construction of a `QuantumGraph`. This identifier will stay constant
through a pickle, and any `QuantumGraph` methods that return a new
`QuantumGraph`.
A `NodeId` will not be the same if a new graph is built containing the same
information in a `QuantumNode`, or even built from exactly the same inputs.
`NodeId`s do not play any role in deciding the equality or identity (hash)
of a `QuantumNode`, and are mainly useful in debugging or working with
various subsets of the same graph.
This interface is a convenance only, and no guarantees on long term
stability are made. New implementations might change the `NodeId`, or
provide more or less guarantees.
Definition at line 39 of file quantumNode.py.