Qn robust estimator of scale (first quartile of interpoint distances $|x_i-x_j|$)

`y=Qn(X,dim)`

example

Rousseeuw P.J. and Croux C., (1993), Alternatives to the median absolute deviation, "Journal of American Statistical Association", Vol. 88, pp. 1273-1283

Croux C. and Rousseeuw P.J.(1992) Time-efficient algorithms for two highly robust estimators of scale, in "Computational Statistics", Vol. 1, eds. Y . Dodge and J. Whittaker, Heidelberg: Physika-Verlag, 41 1-428.