def uniq(*vtrs)
      vecs = vtrs.empty? ? vectors.to_a : Array(vtrs)
      grouped = group_by(vecs)
      indexes = grouped.groups.values.map { |v| v[0] }.sort
      row[*indexes]
    end
def uniq(*vtrs)
      vecs = vtrs.empty? ? vectors.to_a : Array(vtrs)
      grouped = group_by(vecs)
      indexes = grouped.groups.values.map { |v| v[0] }.sort
      row[*indexes]
    end
Return unique rows by vector specified or all vectors

 @param vtrs [String][Symbol] vector names(s) that should be considered

 @example

    => #<Daru::DataFrame(6x2)>
         a   b
     0   1   a
     1   2   b
     2   3   c
     3   4   d
     2   3   c
     3   4   f

    2.3.3 :> df.unique
    => #<Daru::DataFrame(5x2)>
         a   b
     0   1   a
     1   2   b
     2   3   c
     3   4   d
     3   4   f

    2.3.3 :> df.unique(:a)
    => #<Daru::DataFrame(5x2)>
         a   b
     0   1   a
     1   2   b
     2   3   c
     3   4   d