def count
        width = @non_group_vectors.size
        Daru::DataFrame.new([size]*width, order: @non_group_vectors)
      end
Count groups, excludes missing values.
@example Using count
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]).count
# =>
# #<Daru::DataFrame:76900210 @name = 7b9cf55d-17f8-48c7-b03a-2586c6e5ec5a @size = 6>
#                           c          d
# ["bar", "one"]            1          1
# ["bar", "two"]            1          1
# ["bar", "three"]          1          1
# ["foo", "one"]            2          2
# ["foo", "three"]          1          1
# ["foo", "two"]            2          2