add an implementation for splitable datasets that are based on instance
pointers. Can be used for general cross validation that requires
partitions from a dataset.
add doxygen documentation for SplitableDataSets.
Shuffle returns its own dataset (for convenience).
use internal vector directly for copying instance pointers in splits
within SplitableDataSets.
Merge pull request #3 from Ruun/sigma_pi

SplitableDatasets