Helper class for iterating over quanta in a graph.
`QuantumGraph.traverse` method needs to return topologically ordered
Quanta together with their dependencies. This class is used as a value
for iterator, it contains enumerated Quantum and its dependencies.
Parameters
----------
quantumId : `int`
Index of this Quantum, unique but arbitrary integer.
quantum : `~lsst.daf.butler.Quantum`
Quantum corresponding to a graph node.
taskDef : `TaskDef`
Task to be run on this quantum.
dependencies : iterable of `int`
Possibly empty sequence of indices of dependencies for this Quantum.
Prerequisites include other nodes in the graph; they do not reflect
data already in butler (there are no graph nodes for those).
Definition at line 55 of file graph.py.