def only_numerics opts={}
      cln = opts[:clone] == false ? false : true
      arry = numeric_vectors.map { |v| self[v] }

      order = Index.new(numeric_vectors)
      Daru::DataFrame.new(arry, clone: cln, order: order, index: @index)
    end
Return a DataFrame of only the numerical Vectors. If clone: false
is specified as option, only a *view* of the Vectors will be
returned. Defaults to clone: true.