Compute the squares, differences, and square differences of each RP timeseries.
expand_RPs(RPs)A \(T \times 4N\) numeric matrix, with the first \(N\) columns
being the original RPs, the next \(N\) being the differences,
the next \(N\) being the squares, and the last \(N\) being the
squared differences.
A \(T \times N\) numeric matrix, where \(T\) is the number of timepoints and \(N\) is the number of RPs (typically six) to expand.