Added test for isochrone graph topology.
The expected isochrone graphs for each revision in the test should be
provided as a dictionary in an associated yaml file.
Currently onle heuristic lower with depth=1 is being tested.
Also, model clases DirectoryEntry, FileEntry and IsochroneNode were
modified so that they can be compared by equlity and hashed.
Added isochrone graph tests for the remaining heuristics.
Depends on D5822