Add an example script for RoboschoolAtlasForwardWalk-v1

[muupan]

This PR adds an example script to train SAC for RoboschoolAtlasForwardWalk-v1. I cannot find any good published benchmark for this env, but the script can achieve maximum returns of 6500-6600 after 10M steps, which is better than https://github.com/openai/roboschool/blob/master/agent_zoo/RoboschoolAtlasForwardWalk_v1_2017jul.py achieving 6100-6200. I'm not sure how they trained the model.

https://github.com/muupan/chainerrl/tree/atlas/examples/atlas

Since the animation looks good, it is put on the top README as well

https://github.com/muupan/chainerrl/tree/atlas

Resolves #102

[muupan]

Roboschool seems not installed correctly in Travis CI. I'll fix it.

Traceback (most recent call last):
  File "examples/atlas/train_soft_actor_critic_atlas.py", line 249, in <module>
    main()
  File "examples/atlas/train_soft_actor_critic_atlas.py", line 128, in main
    sample_env = make_env(args, process_seeds[0], test=False)
  File "examples/atlas/train_soft_actor_critic_atlas.py", line 35, in make_env
    import roboschool  # NOQA
  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/roboschool/__init__.py", line 112, in <module>
    from roboschool.gym_pendulums import RoboschoolInvertedPendulum
  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/roboschool/gym_pendulums.py", line 1, in <module>
    from roboschool.scene_abstract import SingleRobotEmptyScene
  File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/roboschool/scene_abstract.py", line 12, in <module>
    from roboschool  import cpp_household   as cpp_household
ImportError: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.25' not found (required by /home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/roboschool/.libs/libQt5Core.so.5)
The command "./test_examples.sh -1" exited with 1.

[muupan]

Since I had difficulty in installing roboschool in TravisCI, I modified examples_tests/atlas/test_soft_actor_critic.sh to use Pendulum-v0 instead.

[muupan]

/test

[pfn-ci-bot]

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[ummavi]

LGTM!