def diff(max_lags=1)
          ts = self
          difference = []
          max_lags.times do
            difference = ts - ts.lag
            ts = difference
          end
          difference
        end
Performs the difference of the series.
Note: The first difference of series is X(t) - X(t-1)
But, second difference of series is NOT X(t) - X(t-2)
It is the first difference of the first difference
=> (X(t) - X(t-1)) - (X(t-1) - X(t-2))

== Arguments

* *max_lags*: integer, (default: 1), number of differences reqd.

@example Using #diff

ts = Daru::Vector.new((1..10).map { rand })
# => [0.69, 0.23, 0.44, 0.71, ...]

ts.diff   # => [nil, -0.46, 0.21, 0.27, ...]

@return [Daru::Vector]