Ncvreg wrapper

[azev77]

https://github.com/pnavaro/NonConvexPenalizedRegression.jl

better a wrapper than nothing
@pnavaro

[tlienart]

thanks for that; going by the definition of https://statisticaloddsandends.wordpress.com/2018/07/31/the-scad-penalty/ it should be easy to add to the 'robust' regressions in this package.

I'll have a look, would be nice to run comparisons.