Add API function to create an N-link pendulum

[chrisdembia]

For testing and example purposes, it would be nice if the API had a function like createDoublePendulumModel() that created a double pendulum (like in the README). Right now, there is no way to ask the API to give you a prebuilt model (you have to build one yourself or have an OSIM file). Having a utility function like this could help people get started quicker, and could help with demoing and troubleshooting at workshops.

[tkuchida]

createPlanarPendulumModel(unsigned numPendulums = 1u)?