Change FoldSet input/output to be simple list of IDs

[jdurbin]

The numbers aren't intuitive enough or easy enough for others to use. Change the output to be like:

0 TCGA-29-2431,TCGA-61-1730,TCGA-29-1761,TCGA-29-178
1 TCGA-59-2372,TCGA-24-1616,TCGA-04-1644,TCGA-25-2042
2 TCGA-13-1481,TCGA-04-1646,TCGA-24-2027,TCGA-10-0936

Multiple replications can be had simply by starting the numbering over again, so

0 TCGA-13-1481,TCGA-04-1646,TCGA-24-2027,TCGA-10-0936

would be the beginning of the 2nd repetition of K-fold CV.