Add `setPath` to `PathSpring`, `PathActuator`, `Ligament`, and `Blankevoort1991Ligament`

[nickbianco]

Based on a recent forum post, there is not an obvious way to set a path in PathSpring, PathActuator, Ligament, and Blankevoort1991Ligament with the new path API.

Adding setPath would round out the API given the current getPath and updPath.