def head quantity=5
        select_groups_from :first, quantity
      end
Get the top 'n' groups
@param quantity [Fixnum] (5) The number of groups.
@example Usage of head
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]).head(1)
# =>
# #<Daru::DataFrame:82745170 @name = d7003f75-5eb9-4967-9303-c08dd9160224 @size = 6>
#                     a          b          c          d
#          1        bar        one          2         22
#          3        bar      three          1         44
#          5        bar        two          6         66
#          0        foo        one          1         11
#          7        foo      three          8         88
#          2        foo        two          3         33