Add ARSTAN chronology (re-reddening, pooled autocorrelation) after residual chronology is built

[kanchukaitis]

The ARSTAN chronology is created by applying an autoregressive model to the residual chronology to 're-redden' it, based on a calculation of a common, pooled autoregressive component in the tree-ring data. The spectrum of this chronology falls somewhere between the residual and standard chronologies in terms of low frequency power, typically, but often closer to the standard chronology.

[kanchukaitis]

Current implementation in dplR: https://github.com/OpenDendro/dplR/blob/master/R/chron.ars.R