added scripts for repeated CV

Preliminary scripts for repeated cross validation with a README file.

Here's a summary:
- fit.repeated.5fold.cv.knn.R returns summary of extra-samples
  prediction error estimates by repeated cross-validation (RCV).
  Missing data are imputed by the package impute

- extra.sample.predict.analyze.R returns histograms of prediction
  error estimates by using: 1) root mean squared prediction error (RMSPE),
  2) correlation between predicted and observed values in testing subsets

- fit_functions.R contains several functions including for RCV

- fit_functions_trim.R differs by fit_functions.R for the addition of
  predictors trimming to RCV according to the resolution of linear
  dependencies among predictors (caret package)