def get_group group
        indexes   = groups_by_idx[group]
        elements  = @context.each_vector.map(&:to_a)
        transpose = elements.transpose
        rows      = indexes.each.map { |idx| transpose[idx] }

        Daru::DataFrame.rows(
          rows, index: indexes, order: @context.vectors
        )
      end
Returns one of the selected groups as a DataFrame.
@param group [Array] The group that is to be selected from those grouped.

@example Getting a group

df = Daru::DataFrame.new({
a: %w{foo bar foo bar   foo bar foo foo},
b: %w{one one two three two two one three},
c:   [1  ,2  ,3  ,1    ,3  ,6  ,3  ,8],
d:   [11 ,22 ,33 ,44   ,55 ,66 ,77 ,88]
})
df.group_by([:a, :b]).get_group ['bar','two']
#=>
##<Daru::DataFrame:83258980 @name = 687ee3f6-8874-4899-97fa-9b31d84fa1d5 @size = 1>
#                    a          b          c          d
#         5        bar        two          6         66