Adds a three link conical pendulum example.

This adds the example problem Tarun developed and used when designing the
visualization package. I've updated it to run with the current version of PyDy.
The original pull request can be found here:

https://github.com/pydy/pydy_examples/pull/11
Spelling.
Merge pull request #72 from pydy/conical-pendulum

Adds a three link conical pendulum example.