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.