Investigate differences between Cassie models, and select one

[mposa]

There are currently multiple Cassie models with slightly different parameters.

1. A Gazebo (SDF) simulation from Agility: https://github.com/agilityrobotics/cassie-gazebo-sim
2. A MuJoCo model from OSU: https://github.com/osudrl/cassie-mujoco-sim
3. Kinematic documentation from Agility: https://github.com/agilityrobotics/agility-cassie-doc/wiki/Kinematic-Model
4. A URDF from Harvard: https://github.com/kuindersma/cassie-model
5. The cassie_v2 version currently checked into dairlib

Notable differences include:

• The Gazebo simulation uses <gearbox> elements in SDF, which (1) add extra mass and (2) create a transmission ratio. The associated Gazebo plugin, however, inverts the transmission ratio.
• cassie_v2 is based originally on the Harvard model. Both neglect the plantar linkage. We currently treat the Achilles linkage as a massless distance constraint. The effect of the removed masses should be better compensated for.

Ultimately, we just need to look over these models, understand any differences, and settle on a URDF/SDF to use.

[yminchen]

Short note: cassie_v2 doesn't have effort limits in toe motors