def macd(fast=12, slow=26, signal=9)
          macdseries = ema(fast) - ema(slow)
          macdsignal = macdseries.ema(signal)
          macdhist = macdseries - macdsignal
          [macdseries, macdsignal, macdhist]
        end
Moving Average Convergence-Divergence.
Calculates the MACD (moving average convergence-divergence) of the time
series.
@see https://en.wikipedia.org/wiki/MACD

@param fast [Integer] fast period of MACD (default 12)
@param slow [Integer] slow period of MACD (default 26)
@param signal [Integer] signal period of MACD (default 9)

@example Create a series and calculate MACD values
ts = Daru::Vector.new((1..100).map { rand })
# => [0.69, 0.23, 0.44, 0.71, ...]
macdseries, macdsignal, macdhist = ts.macd
macdseries, macdsignal, macdhist = ts.macd(13)
macdseries, macdsignal, macdhist = ts.macd(signal=5)

@return [Array<Daru::Vector>] macdseries, macdsignal and macdhist are
returned as an array of three Daru::Vectors