This interface defines the contract which every custom feature must implement. Custom features are seem as feature extractors, as they
use input device data to extract features (semantically powerful information).
Computes a feature, according to the current performer position. A drawback of the current implementation is that feature extractors
cannot look into a time window of performer positions, nor to current built-in features (for derivate features computation).
performerPosition - the performer position, from which the feature will be computed.