Compute Z angle

[roaldarbol]

We could make a function that computes the angle relative to the XY plane. It would find the maximal distance between two points, and then by cosine/sine find the angle - e.g. when the points are super close, they must be almost vertical.

Assumptions are that the segments are rigid (i.e. no compression/extension), and we can’t know whether it’s pointing up or down since there could be two solutions.

Probably most useful for top-view tracking. Could be a proxy for a Z coordinate if we constrain the system such that frontal keypoints always go up, and rear keypoints go down (i.e. the animal never points down).

Needs some more thought though.