Provides a strong initialization for flat growth by setting the
growth to 0 and calculates the offset parameter that pass the
function through the mean of the the y_scaled values.
Usage
flat_growth_init(df)
Arguments
df
Data frame with columns ds (date), y_scaled (scaled time series),
and t (scaled time).
Value
A vector (k, m) with the rate (k) and offset (m) of the flat
growth function.