updated editor to work with persistence.  Modified test rail_env / single_agent and re-enabled.  Added test_env_loop.pkl to go with the test.

changed the assertRaises to check for ValueError rather than SystemExit when cities cannot be packed in

added some debug (now commented) looking into the shortest path failures.  Now apparently identified / resolved.

switched back to load_resource in rail_env test. passed load_from_package arg down to from_file generators.  switch default renderer to PGL from BROWSER. Added conditional import for flask_util in rendertools to hopefully avoid some startup delays and hanging tests.  Added os.makedirs to intro_flatland 2.1 for saved frames folder.  Removed explicit gl=PILSVG

trainrun instead of train_run and waypoint instead of way_point for compatibility with Trainrun and Waypoint for readability