0.1.5

This release fixes a number of stale issues and updates the typing annotations in core.

This includes;

• Addition of ExtendObservation wrapper to nest TimeStep fields inside the observation (useful for POMDPs)
• Update all calls to jax.random.PRNGKey to jax.random.key.
• Some typing updates in core to make use of TypeVars. This is still incomplete and tracked in #28.
• Update all chex tests to remove ignore_nones arguments.