Spacing (S): It was proposed by Schott and numerically
measures the diversity of the obtained PF. It is evaluated by
measuring relative distance between neighboring solutions on a PF,
Eq. where, n represents the number of solutions in PF, di is distance
between a solution and the solution i, and d is the average value of
alldi. S ¼ 0 means all solutions are spaced uniformly over PF.
Therefore, it is more desirable to generate a PF with less S value