Arm curl example

[Ian Danforth]

Try to do this without gym dependency.

[Ian Danforth]

This is now done as part of 327902b

Thinking about this more this example is supposed to be a minimal example in the context of OpenAI Gym. The truly minimal example is the one that lives in the README.

While the example I've implemented mimics some of the API of a gym environment, it's not really a useful starting point to further explore the library or set up more detailed experiments.

Doing this first draft also raised the following important question for the API

When you're dealing with multiple muscles does having a container make sense? Should users just keep instances in a list and then step them all? What about when they need to get the values out of each muscle?

I've added these questions to the Revise API ticket.