Generate synthetic mocap data for the swimmer

[jf514]

Towards creating a simple end-to-end test (#2) that can run CI very quickly, it would be good to have simpler model. For this reason, we'll start to support the swimmer model.

To facilitate the introduction of this model, we'll need synthetic mocap data from the swimmer.

Create a mujoco environment, and give each joint the swimmer sinusoidal motion. Define keypoints, and the 3D time series for each keypoint out in a .new file. (Make sure the NWB file has node_names field written into it.)

Note there is no need to support the swimmer model in stac-mjx until we have an understanding of what it takes to support stac-mjx output for a general model.