def percent_change periods=1
          df_numeric = only_numerics.vectors.to_a
          df = Daru::DataFrame.new({}, order: @order, index: @index, name: @name)
          df_numeric.each do |vec|
            df[vec] = self[vec].percent_change periods
          end
          df
        end
The percent_change method computes the percent change over
the given number of periods for numeric vectors.

@param [Integer] periods (1) number of nils to insert at the beginning.

@example

df = Daru::DataFrame.new({
'col0' => [1,2,3,4,5,6],
'col2' => ['a','b','c','d','e','f'],
'col1' => [11,22,33,44,55,66]
},
index: ['one', 'two', 'three', 'four', 'five', 'six'],
order: ['col0', 'col1', 'col2'])
df.percent_change
#=>
#   <Daru::DataFrame:23513280 @rows: 6 @cols: 2>
#              col0                col1
#   one
#   two	   1.0	               1.0
#   three	   0.5                 0.5
#   four	   0.3333333333333333  0.3333333333333333
#   five       0.25                0.25
#   six        0.2                 0.2