Refactor the isochrone graph computation

attempt to simplify a bit this part of the code:

• IsochroneNode are now only used for directories
• FileEntry are used directly from IsochroneNode.entry.files (no need for creating new FileEntry instances), so
• IsochroneNode.children only stores IsochroneNode (thus DirectoryEntry) objects,
• rename IsochroneNode.date as 'dbdate' and clarify its semantics,
• attempt to document (comments) a bit more the algorithm and semantics of several attributes/variables used in there.