Angle tracking and cleanup

[nic-barbara]

We need to add a task for tracking angles, and we need to clean up the existing code/docs to make it easier to use. Remaining things to fix up are:

• [x] Add angle-tracking tasks (generic).
• [x] Add task to Cassie sim to keep pelvis at desired orientation (could be controlled with joystick later).
• [x] See if we still need the joint damping task.
• [x] Tune controller gains (methodically).
• [ ] Update README.md for Cassie examples.

[dazzmo]

New damotion library's control module now addresses these tasks, with demonstrations shown in the cassie examples in 5b90119a7c6fc99693b3371f4994a8659659f221.