attempt to simplify a bit this part of the code:
- IsochroneNode are now only used for directories
- FileEntry are stored in a new IsochroneNode.files attribute, so
- IsochroneNode.children only stores IsochroneNode (thus DirectoryEntry) objects,
- rename IsochroneNode.date as 'dbdate' and clarify its semantics
Also (in dedicated revisions):
- Improve a bit the code of ProvenanceDBBase
- Add str methods to RevisionEntry, DirectoryEntry and FileEntry to ease logging and debugging.
- Add 'ls_files()' and 'ls_dirs()' methods to the DirectoryEntry class to make it a bit easier to compute the isochrone graph (see following revisions).
Depends on D5772