Compare robust and classical ACF

[coatless]

Per SMAC-Group/TTS#18,

From @stephaneguerrier:

@coatless could you please add the functionality we discussed today to compare robust and classical ACF. What I propose it a function ACF(..... , estimator = x), where x can be "classical (default)", "robust" and "both". Then a function plot.ACF if option both is used the compare ACFs.