RLM robust estimator Exact Fit Property

[josef-pkt]

Exact Fit Property: if it is possible to exactly fit at least half of the points, then an estimator with EFP returns the exact fit to these points.

Reference: Maronna, Martin and Yohai 2006 book section 5.10
I have seen it in several parts of the book, in the description of the estimators, there are special cases for scale=0.

see #55 check if we have this,
I think we do after my PR #1341 if scale_est='mad' but not with HuberScale.