Simple example of pydy.codegen and pydy.viz

It would be very useful to have a simple example of pydy.codegen and pydy.viz. This PR provides this example, using the most common multibody example around.

- [x] Document calls to viz classes.
- [x] Fix line wrapping (80 chars).
- [x] Can I remove the `specified` arguments?
- [x] Split `orientnew` and `locatenew` commands across multiple lines.