Estimate the smoothing parameter for the level-alpha and
the smoothing parameter for the trend-beta.
hw_parameters
considers additive seasonal trend: ets(A,A,A) model.
holt_parameters(x)hw_parameters(x)
a univariate time series
holt_parameters
produces a vector of 2 values: alpha, beta.
hw_parameters
produces a vector of 3 values: alpha, beta and gamma.