def emsd(n=10, wilder=false)
          result = []
          emv_return = emv(n, wilder)
          emv_return.each do |d|
            result << (d.nil? ? nil : Math.sqrt(d))
          end
          Daru::Vector.new(result, index: @index, name: @name)
        end
Exponential Moving Standard Deviation.
Calculates an exponential moving standard deviation of the series using a
specified parameter. If wilder is false (the default) then the EMSD
uses a smoothing value of 2 / (n + 1), if it is true then it uses the
Welles Wilder smoother of 1 / n.

@param [Integer] n (10) Loopback length.
@param [TrueClass, FalseClass] wilder (false) If true, 1/n value is
used for smoothing; if false, uses 2/(n+1) value

@example Using emsd

ts = Daru::Vector.new((1..100).map { rand })
# => [0.400..., 0.727..., 0.862..., 0.013..., ...]

# first 9 observations are nil
ts.emsd   # => [ ... nil, 0.285... , 0.258..., 0.243..., ...]

@return [Daru::Vector] contains EMSD