fixed bug with the dimensions when method==rpc-based, apply_to==sign, cross_validation==TRUE and the correlation is not significant

added na.rm, apply_to and alpha parameters and their checks. Changed na.rm=T for na.rm=na.rm everywhere. Added documentation.

1) Introduction of ncores as extra parameter. 2) Introduction of Calibration as a new function (replacement of .calibration.wrap). 3) removal of var.fc

Handling of case of insufficient data, following the comment of Deborah Verfaillie. The function now warns (and does no calibration) when 3 or less observation-model pairs are available for training. A new parameter na.fill has been introduced.

Removing aperm manipulations before the Apply function in CST_Calibration.R and partly reversing corrections of Verónica.

Suggestions of Verònica implemented. More specifically: 1) implementation of multiApply as wrapper, 2) removal of ... 3) warning when multi.model is used, 4) adaptation of the manual by addition of bias method, discussion of use of multi.model boolean, change of adequate reference to 2011 paper for evmos method.