Method ElasticNet

src/app/linear_method/penalty.h:30–33

class ElasticNet : public Penalty<T> {
 public:
  ElasticNet(T lambda1, T lambda2) : lambda1_(lambda1), lambda2_(lambda2) {
    CHECK_GE(lambda1, 0);
    CHECK_GE(lambda2, 0);
  }
  ~ElasticNet() { }

  T eval(const MatrixPtr<T>& model) { return 0; }  // TODO