Add wrappers for saving and loading brax PPO networks

[vincekurtz]

Resolves #29.

• Implements a simple pickle-able MLP
• Adds a wrapper class for brax's PPONetworks
  • Makes it more obvious how to define custom architectures
  • Makes it easier to save and load the networks along with their parameters
• Adds an example of saving and loading policies for the pendulum

Edit from @alberthli: also resolves #28.