SPLINE Cubic spline data interpolation.
    PP = SPLINE(X,Y) provides the piecewise polynomial form of the
    cubic spline interpolant to the data values Y at the data sites X,
    for use with the evaluator PPVAL and the spline utility UNMKPP.
    X must be a vector.
    If Y is a vector, then Y(j) is taken as the value to be matched at X(j),
    hence Y must be of the same length as X  -- see below for an exception
    to this.
    If Y is a matrix or ND array, then Y(:,...,:,j) is taken as the value to
    be matched at X(j),  hence the last dimension of Y must equal length(X) --
    see below for an exception to this.