State Stacking & Banan Environment

[awjuliani]

• Add support for stacking past n states to allow network to learn temporal dependencies.
• Add Banana Collector environment for demonstrating partially observable multi-agent environments.
• Add 3DBall Hard which lacks velocity information in state representation. Used as test for LSTM and state-stacking features.
• Rework Tennis environment to be continuous control and trainable in 100k steps.