The Greville abscissae are defined to be the mean location of k-1
consecutive knots in the knot vector for each basis spline function of order
k. With the first and last knots in the
knot vector excluded, there are
gsl_bspline_ncoeffs Greville abscissae
for any given B-spline basis. These values are often used in B-spline
collocation applications and may also be called Marsden-Schoenberg points.
Returns the location of the i-th Greville abscissa for the given B-spline basis. For the ill-defined case when k=1, the implementation chooses to return breakpoint interval midpoints.