def mean
        apply_method :numeric, :mean
      end
Calculate mean of numeric groups, excluding missing values.
@example Usage of mean
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]).mean
# =>
# #<Daru::DataFrame:81097450 @name = 0c32983f-3e06-451f-a9c9-051cadfe7371 @size = 6>
#                         c          d
# ["bar", "one"]          2         22
# ["bar", "three"]        1         44
# ["bar", "two"]          6         66
# ["foo", "one"]        2.0       44.0
# ["foo", "three"]        8         88
# ["foo", "two"]        3.0       44.0