Add scaling method (sensu Esper et al. 2005)

[cszang]

Scaling results in homogeneous variability between the instrumental target and proxy, at the expense of inflated error variance. Still, this method is used and should be incorporated into treeclim.

Key question: where? This will probably only be interesting for the skills() related aspects.

[catalinroibu]

Hi Christian,

skills() in the perfect place to insert this desired new option for climate reconstruction.

All the best!