def pos *indexes
      if indexes.first.is_a? Integer
        return indexes.first if indexes.size == 1
        return indexes
      end
      res = self[indexes]
      return res if res.is_a? Integer
      res.map { |i| self[i] }
    end
Returns positions given indexes or positions
@note If the arugent is both a valid index and a valid position,
it will treated as valid index
@param indexes [Array<object>] indexes or positions
@example
idx = Daru::MultiIndex.from_tuples [[:a, :one], [:a, :two], [:b, :one], [:b, :two]]
idx.pos :a
# => [0, 1]