OPTrho computes rho function for optimal weight function

`rhoOPT=OPTrho(u, c)`

example

Maronna, R.A., Martin D. and Yohai V.J. (2006), "Robust Statistics, Theory and Methods", Wiley, New York.

Riani, M., Cerioli, A. and Torti, F. (2014), On consistency factors and efficiency of robust S-estimators, "TEST", Vol. 23, pp. 356-387.

Yohai V.J., Zamar R.H. (1997) Optimal locally robust M-estimates of regression. "Journal of Planning and Statistical Inference", Vol. 64, pp. 309-323.