Skeleton handling

[roaldarbol]

We need to figure out how skeletons are handled, and which methods are needed for them.

Some ideas:

• Goes into metadata$skeleton
• set_skeleton - can either be explicit or maybe a path to a config file from DLC or SLEAP?
• get_skeleton
• plot_pose - some way of showing a frame with lines between connected dots. Maybe make a GIF or plotly thing if multiple frames are chosen?
• filter_na_pose (or what it was I called it) to replace poor values; figure out how the skeleton will fit into it.

[roaldarbol]

This bit is also important for calculating angles between keypoints (maybe?). It's at least a limitation of which angles should be calculated for each point (being the vertex).

[roaldarbol]

Should also check that the keypoint actually exist in the data.