Added support for AUC as metric for cross-validation

Added future to imports

Added two checks for valid methods

Changed plot.sNPLS functions from base to ggplot2. Most arguments of the functions have been mantained, so this should not produce a major break in compatibility with code from previous versions.

Added unfolded X (Xd) to the output of sNPLS

Added more options for the function plot.sNPLS()