GitLab

made the env loading more forgiving (in persistence) to allow the loading of old envs, and msgpack files with .pkl extension.  Also give default 100 max_episode_steps where this is missing.  Add test-saved-envs notebook to test loading all old envs.

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