'Returns an iterator that iterates over all dependency relations.
>>> tracker = DependencyTracker()
>>> tracker.add(0, 1)
>>> tracker.add(0, 2)
>>> tracker.add(1, 2)
>>> for dependency, dependant in sorted(tracker):
...     print(dependency, \'->\', dependant)
0 -> 1
0 -> 2
1 -> 2'

def __iter__(self):

    return ((dependency, dependant) for (dependency, dependants) in self._dependency_dict.items() for dependant in dependants)
