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