Convert Numpy Docstrings to Google Style

[frederikschubert]

This issue tracks the progress of converting the numpy style docstrings to the more concise Google style.

• [ ] _core
• [ ] experience_replay
• [ ] model_updaters
• [ ] policy_objectives
• [ ] proba_dists
• [ ] reward_tracing
• [ ] td_learning
• [ ] utils
• [ ] value_transforms
• [ ] wrappers

This depends on the type annotations https://github.com/coax-dev/coax/issues/13 for easier automatic conversions.