Bug Fixes and Improvements (VecEnv)

Warning: breaking change when using custom policies

• doc update (fix example of result plotter + improve doc)
• fixed logger issues when stdout lacks read function
• fixed a bug in common.dataset.Dataset where shuffling was not disabled properly (it affects only PPO1 with recurrent policies)
• fixed output layer name for DDPG q function, used in pop-art normalization and l2 regularization of the critic
• added support for multi env recording to generate_expert_traj (@XMaster96)
• added support for LSTM model recording to generate_expert_traj (@XMaster96)
• GAIL: remove mandatory matplotlib dependency and refactor as subclass of TRPO (@kantneel and @AdamGleave)
• added get_attr(), env_method() and set_attr() methods for all VecEnv.
  Those methods now all accept indices keyword to select a subset of envs.
  set_attr now returns None rather than a list of None. (@kantneel)
• GAIL: gail.dataset.ExpertDataset supports loading from memory rather than file, and
  gail.dataset.record_expert supports returning in-memory rather than saving to file.
• added support in VecEnvWrapper for accessing attributes of arbitrarily deeply nested
  instances of VecEnvWrapper and VecEnv. This is allowed as long as the attribute belongs
  to exactly one of the nested instances i.e. it must be unambiguous. (@kantneel)
• fixed bug where result plotter would crash on very short runs (@Pastafarianist)
• added option to not trim output of result plotter by number of timesteps (@Pastafarianist)
• clarified the public interface of BasePolicy and ActorCriticPolicy. Breaking change when using custom policies: masks_ph is now called dones_ph.
• support for custom stateful policies.
• fixed episode length recording in trpo_mpi.utils.traj_segment_generator (@GerardMaggiolino)