add metric adjusted r2

[behrica]

I came with this implementation;

(defn adj-r2 [lhs rhs p]
  (let [r2 (stats/r2 lhs rhs)
        n (count lhs)]
    (- 1
       (* (- 1 r2)
          (/ (dec n) (- n p 1))))))

from here:
https://en.wikipedia.org/wiki/Coefficient_of_determination#Adjusted_R2

[genmeblog]

There is an omega-sq which is adjusted R2, I can't match results, something is wrong with it I suppose.

[genmeblog]

Ok, I will add a third argument for degrees of freedom (features) to adjust r2. It will be in 3.x.