def max
      assert_ordered :max
      categories.last
    end
Returns the maximum category acording to the order specified.
@note This operation will only work if vector is ordered.
To set the vector ordered do `vector.ordered = true`
@return [object] the maximum category acording to the order
@example
dv = Daru::Vector.new ['second', 'second', 'third', 'first'],
categories: ['first', 'second', 'third']
dv.max
# => 'third'