Reorganize code
Moved isochrone graph logic to its own file graph.py. Origin-revision layer's algorithm is now in origin.py, while revision-content layer's logic was moved to revision.py.
Migrated from D5849 (view on Phabricator)
Moved isochrone graph logic to its own file graph.py. Origin-revision layer's algorithm is now in origin.py, while revision-content layer's logic was moved to revision.py.
Migrated from D5849 (view on Phabricator)