This custom feature computes the angle between the head and the performer feet. If only one feet is visible by the input device, it is
used to compute the final value. If both feet are visible, the average of both points is taken into the computation. Returns a normalised
performerPosition - the performer position, from which the feature will be computed.
a value between zero and 1. If zero is returned, it means the person is exactly straight in the vertical position.
Values next to 1 mean the person is upside down. Values next to 0.5 mean an horizontal position.